Contractile Response of Horse Deep Dorsal Penile Vein to Histamine

Contractile response of horse deep dorsal penile vein to histamine

AC Martıńez1, D Prieto1, M Hernańdez1, A Garcıá-Sacristań1 and S Benedito1*

1Seccio´n Departamental de Fisiologı´a, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain

The present investigation was designed to evaluate the effect of histamine on isolated rings of horse deep dorsal penile vein. Under precontracted or basal conditions, histamine evoked an endothelium-independent contraction. Preincubation of the vein rings with the selective H1 receptor antagonist, mepyramine, shifted the concentration – response curve for histamine and to the H1 receptor agonist 2-pyridylethylamine to the right in a competitive manner. Pretreatment with cimetidine, a speciﬁc H2 receptor antagonist, did not modify the pEC50 and maximal contraction of the histamine response. Cimetidine and propranolol failed to induce a change in the relaxation caused by dimaprit, the H2 receptor agonist. Histamine contraction was unaffected by thioperamide, the speciﬁc H3 receptor antagonist. (R)-a-methylhistamine, the H3 receptor agonist, also induced contractions which persisted in the presence of either thioperamide or tetrodotoxin. These data indicate that horse deep dorsal penile vein shows an endothelium-independent contraction response to histamine, mainly mediated by H1 receptors. International Journal of Impotence Research (2002) 14, 85–92. DOI: 10.1038=sj=ijir=3900830

Keywords: deep dorsal penile vein; histamine; histamine receptors; H1 receptor; penile erection

Introduction treatment of impotency,5,6 the role played by vasoactive factors such as cytokines, autacoids and=or neurotransmitters in maintaining normal The integrity of the vasculature of the penis is penile function is not yet fully understood. Post- essential for normal erectile function, with vascular junctional a-andb-adrenoceptors and prejunctional damage being one of the most common pathological a2-adrenoceptors have been identified in corpus 1 causes of erectile dysfunction. Under both normal cavernosum7 and in penile resistance (helicine) and pathological conditions, pressure buildup in the arteries, which control blood flow between the corpus cavernosum during erection depends mainly systemic arterial circulation and cavernous sinu- on the interaction between the arterial inflow system soids.8,9 In circumflex veins10 and in the deep dorsal and the venous draining system. It should be penile vein,11 a-adrenoceptor functions have also pointed out that venous draining vessels do not been demonstrated. Histamine has also been pro- fully collapse and flow through the penis continues posed as a possible mediator of penile erection on 2 throughout the erection period. The active con- the grounds that histamine-containing mastocytes tribution of penile veins to the reduction in venous have been identified in erectile tissue, coupled with outflow has not been fully established, but the the fact that the administration of this amine to the findings of several studies suggest that an active cavernous body is capable of evoking an erec- mechanism of venous occlusion adds to passive tion.12,13 Kirkeby et al,10 however, found that the compression of the outflow pathway resulting from histamine-induced relaxation of precontracted hu- 1,3 cavernous sinusoidal distension. Moreover, some man corpus cavernosum preparations and circum- authors claim that large veins including the deep flex veins was not significantly affected by blockage dorsal veins might be involved in the physiology of 14 of H1 and H2 receptors. In contrast, Kim et al, erection.4 reported that the histamine H1 receptor was the Though the discovery of the selective and potent main functional receptor subtype mediating smooth inhibitor of type V cyclic GMP phosphodiesterase, muscle contraction in the rabbit corpus cavernosum, sildenafil, has meant considerable advance in the yet other authors have shown the involvement of H2 receptors and no evidence of H1 receptor function in the histamine-induced relaxation of human caver- nosal strips.15 Moreover, our recent observations *Correspondence: S Benedito, Seccioń Departamental de Fisiologıá, Facultad de Farmacia, Universidad Complutense, indicate that the effect of histamine on dorsal penile 28040 Madrid, Spain. arteries is quite variable among different species E-mail: [email protected] (human, horse and rabbit).16 – 18 This amine has been Received 27 August 2001; accepted 21 November 2001 shown to elicit vasodilation, vasoconstriction, or a Contractile response of horse deep dorsal penile vein to histamine AC Martıńez et al 86 combination of both these responses via stimulation (1074 M), was interpreted to indicate a non-func- of H1 receptors, H2 receptors or both. Taken tional endothelium. together, the results of these studies point to a role For experiments in which the effect of agonists on of this endogenous amine in the regulation of penile vein rings precontracted with noradrenaline function. (1076 M) (2.58 Æ 0.21 g; n ¼ 30) were examined, the This study was designed to determine the results are given as a percentage of the vascular response of the horse deep dorsal penile vein to contraction induced by this preconstrictor. These histamine, to characterise the functionally active noradrenaline concentrations yielded a stable level histamine receptors and to examine the dependence of contraction of sufficient duration to permit the of the histamine-induced response on the presence analysis of the agonist response. Effects observed in of the endothelial layer. preparations at basal tone were expressed as a percentage of the contractile response to K-PSS shown by each vessel (4.30 Æ 0.33 g; n ¼ 30). Materials and methods Since consecutive cumulative concentration – response curves to histamine indicated a desensiti- sation effect that modified the agonist response; only Vascular preparations and measurement of tension one concentration – response curve per vein ring was obtained under precontracted and basal conditions. Thus, it was necessary to use consecutive The penile tracts of young horses were obtained segments from the same animal in parallel experi- from a local slaughterhouse shortly after they were ments, with one ring acting as the control for the killed. The entire penis was transported to the other. Cumulative concentration – response curves laboratory in ice-cold physiological saline solution for histamine and various histaminergic agonists (PSS) to minimise tissue metabolism. were obtained by increasing the organ bath concen- The deep dorsal vein was identified and dissected tration in half-log-unit steps. free from surrounding connective tissue with the aid In order to examine the activity of each individual of a stereomicroscope (Nikon SMZ 2B). Vessel rings histamine receptor subtype, a given receptor agonist approximately 3 mm in length were transferred to was tested after pretreatment with a specific antago- organ baths containing PSS at 37C and aerated with nist to the other two receptor subtypes. a mixture of 95% O2 and 5% CO2 to maintain the pH at 7.4. The rings were mounted between two parallel L-shaped stainless steel wires (150 mm diameter). Analysis of results The endothelium of some rings was mechanically removed by gentle rubbing of the intimal surface with a stainless steel wire. Special care was taken to The agonist concentration required to give a half- avoid damage to the endothelium. One wire was maximal response (EC50) was determined for each fixed to a displacement unit allowing fine adjust- concentration – response curve by computerised ment of tension while the other was attached to a iteration. Responses and logarithmic concentrations force transducer (Grass FT03C). Isometric tension were fitted to the Hill equation using the GraphPAD was recorded on a polygraph (Houston D-5236-5). InPlot, non-linear curve fitting programme for The preparations were allowed to equilibrate for personal computers (GraphPAD Software, San Die- about 30 min in PSS and washed with fresh (37C) go, CA, USA). Sensitivities to the agonists are given PSS at 15 min intervals. After this equilibration in terms of pEC50 values, defined as the negative period, each ring was stretched in a stepwise logarithm of the EC50 for the agonists used. fashion to the optimal point of its length-tension Antagonist potency was assessed by calculating ratio ( 1 g). the pA2 value determined from the Schild plot analysis.19 In this method, the log concentration ratio (CR)-1 of the agonist is plotted against the log Experimental procedure concentration of the antagonists and these plots are then subjected to linear regression analysis. The concentration ratio denotes the ratio of the EC50 In all the experiments, the vein rings were exposed value obtained in the presence and absence of a to a twice depolarising potassium solution (119 mM, given concentration of antagonist. The slope of the K-PSS) at intervals of about 30 min. Schild plot is an indication of the nature of the The integrity of the vascular endothelium was antagonism. The slope should ideally be 1 if the confirmed by immediate relaxation (80 – 100%) response involves simple competitive antagonism at induced by acetylcholine (1076 M) in vessels pre- one receptor subtype only. Provided the regression contracted by noradrenaline. A lack of relaxation of the Schild plot is linear and the slope is not induced by exposure to the same concentration of significantly different from unity, the antagonist is acetylcholine with no loss of response to papaverine in equilibrium with the receptor and the pA2 value,

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 87 which is the intercept along the abscissa scale of the pyridylethylamine dihydrochloride (a kind gift from Schild plot, is equal to the negative logarithm of the SmithKline Beecham, UK), (R)-a-methylhistamine equilibrium dissociation constant for the antagonist: dihydrogenomaleate and thioperamide maleate (a pA2 ¼ 7 logKB ¼ pKB. In this case, the points were kind gift from Bioproject, France). All drugs were constrained to a slope of unity and a more precise added in volumes not exceeding 0.3% of that of the 20 value of the pKB was calculated. organ bath to reach the final required concentration, and were dissolved in distilled water with the following exceptions: papaverine were prepared in Solutions and drugs ethanol (99%) and thioperamide was dissolved in dimethylsulphoxide. Previous experiments showed that the solvents used had no effect on preparations. The vessels were dissected and kept relaxed in PSS Stock solutions of drugs were stored at 7 20C and of the following composition (mM): NaCl 119, KCl fresh dilutions were made daily. Concentrations of 4.7, CaCl2 1.5, MgSO4 1.2, NaHCO3 25, glucose 10, agents were expressed as the final concentration in KH2PO4 1.2 and ethylene-diaminetetra-acetic acid the organ bath. (EDTA) 0.026. K-PSS was identical to PSS except that NaCl was replaced with KCl on an equimolar basis. Statistical analysis The following drugs were used: acetylcholine hydrochloride, cimetidine, histamine dihydrochloride, noradrenaline hydrochloride, papaverine Results are given as mean values Æ standard error of hydrochloride, phentolamine hydrochloride, pro- the mean (s.e.m.) or 95% confidence limits. Statis- pranolol hydrochloride, tetrodotoxin (Sigma, USA), tical determinations were performed using the mepyramine maleate (a kind gift from Rhoˆne Student’s t-test for paired or unpaired observations Poulenc Rorer, UK), dimaprit dihydrochloride, 2- where appropriate. A P-value less than 5%

Figure 1 (a) Effect of histamine H1 receptor antagonist on histamine concentration – response curves in horse deep dorsal penile veins at resting tone. (b) Schild plot for the effect of mepyramine on the concentration – response curves for histamine. Each point represents the mean response Æ s.e.m. shown by seven vessel rings.

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 88 (P < 0.05) was taken to denote a significant and after the addition of the antagonist, difference. pEC50 ¼ 6.04 Æ 0.08; MC ¼ 116.80 Æ 13.19%; n ¼ 8; Figure 2a). Furthermore, after selective antagonism by mepyramine (1077 M), histamine failed to pro- Results duce a relaxation of the dorsal vein and this antagonist only induced a significant shift of the curve to the right with no further change in the Contractile response of horse deep dorsal penile maximal contraction (control, pEC50 ¼ 6.20 Æ 0.07; vein to histamine MC ¼ 106.80 Æ 14.29%; and after the addition of the antagonist, pEC50 ¼ 4.19 Æ 0.12, P < 0.05; MC ¼ 100.52 Æ 15.13%; n ¼ 6; Figure 2b). 79 73 Histamine (10 – 10 M) caused a concentration- Selective H3 receptor inhibition with thiopera- dependent contraction both in rings contracted with mide (1076 M) did not affect the histamine-induced 76 noradrenaline (10 M) (pEC50 ¼ 6.11 Æ 0.05; maxi- contraction of the deep dorsal vein at basal tone, mal contraction (MC) ¼ 105.75 Æ 7.70%; n ¼ 14), and previously blocked with the H1 and H2 antagonists. under conditions of resting tone There was no significant difference in the values of (pEC50 ¼ 6.38 Æ 0.11; MC ¼ 148.43 Æ 6.23%; n ¼ 8). pEC50 and maximal contraction before and after Mechanical removal of the endothelium did not treatment with thioperamide (control, pEC50 ¼ significantly affect the histamine concentration – 4.81 Æ 0.09; MC ¼ 149.36 Æ 8.91%; and after the response curve, both in rings contracted with addition of the antagonist, pEC50 ¼ 4.97 Æ 0.07; 76 noradrenaline (10 M) (pEC50 ¼ 6.14 Æ 0.07; maxi- MC ¼ 140.23 Æ 7.53%; n ¼ 7). mal contraction (MC) ¼ 102.28 Æ 4.08%; n ¼ 14), and under conditions of resting tone (pEC50 ¼ 6.27 Æ 0.09; MC ¼ 158.84 Æ 9.75%; n ¼ 8). Histamine receptor agonists We also found histamine only induces contraction in horse corpus cavernosum strips (data not shown). 2-Pyridylethylamine (PEA, 1079 – 1073 M), a selective histamine H1 receptor agonist, provoked a Histamine receptor antagonists concentration-dependent contraction of resting vessel rings (pEC50 ¼ 4.70 Æ 0.05; MC ¼ 100.18 Æ 8.16%; n ¼ 18). Mepyramine (3 6 1079 – 3 6 1078 M) To rule out the involvement of adrenoceptors in the caused a concentration-dependent rightward shift histamine-induced contraction, a set of experiments in the PEA response (Figure 3a), showing a pA2 was performed on resting vein rings in the presence value of 8.68 Æ 0.19 and a slope not significantly of phentolamine (1076 M). Inhibition of a-adreno- different to unity (0.98 Æ 0.22) (Figure 3b). The plots ceptors did not affect the concentration response were constrained to unity yielding a pKB value of curves to histamine (control, pEC50 ¼ 6.43 Æ 0.22; 8.30 Æ 0.21 for mepyramine (n ¼ 6). 79 73 MC ¼ 139.33 Æ 8.48%; and after the addition of the Dimaprit (10 – 10 M), a specific histamine H2 antagonist, pEC50 ¼ 6.26 Æ 0.10; MC ¼ 143.32 Æ receptor agonist, showed a relaxant effect 7.20%; n ¼ 8). (pEC50 ¼ 3.62 Æ 0.05; maximal relaxation (MR) ¼ To determine whether the H1 receptor is involved 46.19 Æ 4.61%) at very high concentrations on in the contractile effect shown by histamine, con- precontracted veins. The presence of cimetidine centration – response curves of resting segments (1075 M) in the organ bath did not affect the control were obtained in the presence of increasing con- response to dimaprit (pEC50 ¼ 3.51 Æ 0.08; MR ¼ centrations of mepyramine (3 6 1079 – 3 6 1078 M). 40.65 Æ 3.95%; n ¼ 11). Furthermore, the relaxant Figure 1a shows that the H1 receptor antagonist response was not inhibited by propranolol induced a parallel rightward displacement of the (3 6 1076 M), a b-adrenoceptor antagonist, since no curves without changing maximal contractions. The modification of the dimaprit effect was observed Schild plot yielded a line with a slope of 0.98 Æ 0.16 (control, pEC50 ¼ 3.46 Æ 0.05; MR ¼ 32.90 Æ 7.65%; and a pA2 value of 9.10 Æ 0.12 (Figure 1b). The pKB and after the addition of the antagonist, value for mepyramine calculated from a plot con- pEC50 ¼ 3.38 Æ 0.02; MR ¼ 39.03 Æ 6.21%; n ¼ 6). In strained to unity was 8.98 Æ 0.17 (n ¼ 7). addition, this H2 antagonist induced no effect under Given the possibility of unmasking a possible conditions of resting tone. relaxant effect brought about by the H2 or H1 In the presence of the H1 and H2 receptor receptor, the histamine response was tested in antagonists, selective activation of histamine H3 precontracted segments. In the presence of the H2 receptors by the agonist (R)-a-methylhistamine receptor antagonist, cimetidine (1075 M), the con- (1079 – 3 6 1074 M) caused a contractile effect traction induced by histamine did not differ sig- (pEC50 ¼ 3.93 Æ 0.10; MC ¼ 49.66 Æ 7.52%) in resting nificantly from that observed in control responses rings. Thioperamide (1076 M) failed to modify both (control, pEC50 ¼ 6.04 Æ 0.06; MC ¼ 104.90 Æ 8.15%; vessel sensitivity to the H3 agonist (pEC50 ¼

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 89

Figure 2 (a) Effects of histamine H2 and (b) H1 receptor antagonists on the histamine concentration – response curves in horse deep dorsal penile veins precontracted with noradrenaline. Each point represents the mean response Æ s.e.m. shown by eight and six vessel rings, respectively.

4.17 Æ 0.05) and maximal contraction (MC ¼ 42.43 Discussion Æ 5.32%; n ¼ 6). This effect was found to be resistant to tetrodotoxin (1076 M) (control, The main purpose of the present study was to pEC50 ¼ 4.22 Æ 0.09; MC ¼ 40.00 Æ 5.66%; and after identify and characterise functional postjunctional the addition of the Naþ channel blocker, histamine receptors in the deep dorsal penile vein of pEC50 ¼ 4.30 Æ 0.06; MR ¼ 38.20 Æ 4.28%; n ¼ 6). the horse. Histamine produced an endothelium-

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 90

Figure 3 (a) Effect of histamine H1 receptor antagonist on the PEA concentration – response curves in horse deep dorsal penile veins at resting tone. (b) Schild plot for the effect of mepyramine on the concentration – response curves for PEA. Each point represents the mean response Æ s.e.m. shown by six vessel rings.

independent contraction in precontracted vein rings present study is consistent with published data and those at baseline tone. It has been established obtained in functional experiments aimed at identi- 22 that the reactivity of horse erectile tissue is similar fying H1 receptors. to that of man,7 – 9 thus horse penile vessel prepara- As previously noted by our group in rabbit dorsal 18 tions are convenient models for this type of study. penile artery, it is possible that weak H2 receptor- However, the results of the present study suggest mediated relaxant action was masked by the potent that there are species differences with respect to the H1 receptor-mediated contractile effect. However, 10 histamine response in erectile tissue. the presence of an H1 receptor antagonist did not Schoeffter and Godfraind21 suggested that, at high reveal the relaxation effect of this amine. Moreover, histamine concentrations, contractions induced in the selective H2 receptor antagonist, cimetidine, had rat isolated aorta were probably mediated by a- no influence on the histamine-induced contraction. adrenoceptors. In our experiments, any interaction This suggests that it is unlikely that H2 receptors are between histamine and a-adrenoceptors was ruled involved in this histamine response. On the other out since no effect was shown by phentolamine on hand, stimulation of H2 receptors by dimaprit the concentration – response curve for histamine induced a relaxation at high concentrations in recorded in resting penile veins. precontracted preparations but the inability of its The concept of H1 receptor-mediated contraction selective antagonist to block the response suggests of horse vein rings is supported by the mepyramine- the involvement of the H2 receptor is unlikely. induced rightward shift of the concentration – Moreover, any interaction with b-adrenoceptors was response curves obtained. The displacement curve ruled out by the lack of effect of propranolol on the for mepyramine (pA2 of 9.10 and pKB of 8.98) and dimaprit response. These results are in agreement the finding of a slope close to unity indicates with previous findings in studies performed on competitive antagonism. PEA also produced a other preparations, in which H2 receptors are not concentration-dependent contraction that was an- involved in the histamine response.17 tagonised in a competitive fashion by mepyramine H3 receptors, initially identified as presynaptic (pA2 of 8.68 and pKB of 8.30). The pharmacological autoreceptors located on the axon terminals of profile of the histamine receptors examined in the central histaminergic neurons,23 may also act as

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 91 presynaptic heteroreceptors, both in the central and References peripheral nervous system.22 Although this receptor subtype is mainly found at nerve endings, H3 receptors are also known to occur postjunction- 1 Nehra A, Goldstein I, Pabby A. Mechanisms of venous leakage: 24,25 a prospective clinicopathological correlation of corporal ally. However, the presence of histamine H3 function and structure. J Urol 1996; 156: 1320 – 1329. receptors in dorsal penile veins seems unlikely 2 Borowitz E, Barnea O. Hemodynamic mechanisms of penile since thioperamide was unable to alter the contrac- erection. IEEE Trans Biomed Eng 2000; 47: 319 – 326. tile response to histamine. Moreover, according to 3 Carati CJ, Creed KE, Keogh EJ. Vascular changes during erection in the dog. J Physiol 1988; 400:75– 88. our previous results obtained in horse dorsal penile 4 Esen AA et al. Contractility changes of the deep dorsal penile 17 arteries, activation of histamine H3 receptors with vein due to serotonin. J Urol 1997; 158: 234 – 237. (R)-a-methylhistamine provokes a contractile effect 5 Ballard SA et al. Effects of sildenafil on the relaxation of that is resistant to the action of thioperamide. The human corpus cavernosum tissue in vitro and on the activities absence of an effect of tetrodotoxin on agonist- of cyclic nucleotide phosphodiesterase isoenzymes. J Urol 1998; 159: 2164 – 2171. induced contraction noted here, indicates that this 6 Simonsen U, Contreras J, Garc´ıa-SacristańA,Mart´ınez AC. contraction is not an indirect mechanism involving Effect of sildenafil on non-adrenergic non-cholinergic neuro- neuronal transmission, and may possibly be transmission in bovine penile small arteries. Eur J Pharmacol ascribed to a non-specific effect of the agonist. 2001; 412: 155 – 169. 7 Recio P, Lo´pez PG, Fernańdez JLG, Garc´ıa-SacristańA. If we compare the present results with those Pharmacological characterization of adrenoceptors in horse previously obtained in specimens of dorsal penile corpus cavernosum penis. J Auton Pharmacol 1997; 17: 199 – artery of the same species,17 it appears that the 206. 8 Simonsen U et al. Prejunctional alpha 2-adrenoceptors inhibit receptor subtype involved is the same, the H1 receptor. However, this receptor was found to nitrergic neurotransmission in horse penile resistance arteries. J Urol 1997; 157: 2356 – 2360. mediate two different effects; a biphasic response 9 Simonsen U et al. Adrenoceptor-mediated regulation of the (relaxation=contraction) in precontracted arterial contractility in horse penile resistance arteries. J Vasc Res segments and a purely contractile response in vein 1997; 34:90– 102. specimens. Taken together, these results could 10 Kirkeby HJ, Fahrenkrug J, Holmquist F, Ottesen B. Effects of noradrenaline, 5-hydroxytryptamine and histamine on human explain the erection-evoking effect shown when penile cavernous tissue and circumflex veins. Int J Impot Res 12,13 histamine is administered in vivo. 1989; 1: 181 – 188. In general, the contractile response to histamine 11 Fontaine J, Schulman CC, Wespes E. Postjunctional alpha-1 and alpha-2-like activity in human isolated deep dorsal vein of appears to involve the activation of the H1 receptor alone and is unaffected by mechanical removal of the penis. Br J Pharmacol 1987; 89: 493 – 499. 17,18 12 Adaikan PG, Lau LC, Ratnam SS. Physio-pharmacology of the endothelium. However, experiments in human penile erection-autonomic=nitrergic neurotransmis- bovine intrapulmonary vein,26 and equine colonic sion and receptors of the human corpus cavernosum. Asian veins27 provide evidence for the involvement of Pac J Pharmacol 1991; 6: 213 – 217. histamine in mediating endothelium-dependent 13 Andersson KE, Wagner G. Physiology of penile erection. Physiol Rev 1995; 75: 191 – 236. contraction. In our case, the contractile effect of 14 Kim YC et al. Characterization and function of histamine histamine was endothelium-independent and H1 receptors in corpus cavernosum. J Urol 1995; 153: 506 – 510. receptors on vascular smooth muscle mediated 15 Cara AM et al. The role of histamine in human penile erection. direct constriction. These results only partly con- Br J Urol 1995; 75: 220 – 224. firm those obtained in the horse dorsal penile 16 Mart´ınez AC et al. Endothelium-independent relaxation 17 induced by histamine in human dorsal penile artery. Clin artery, in which only the relaxant phase required Exp Pharmacol Physiol 2000; 27: 500 – 507. the presence of an intact endothelium, and the 17 Mart´ınez AC et al. Evidence of histamine receptor function in contractile phase was endothelium-independent. isolated horse penile dorsal arteries. Life Sci 2000; 67: 1355 – According to our present and previous findings, it 1368. 18 Mart´ınez AC, Garc´ıa-Sacristań A, Rivera L, Benedito S. appears that histamine, whether secreted under Biphasic response to histamine in rabbit penile dorsal artery. physiological or pathological conditions, may affect J Cardiovasc Pharmacol 2000; 36: 737 – 743. penile vascular tone and, in turn, influence the 19 Arunlakshana O, Schild HO. Some quantitative uses of drug process of erection. antagonism. Br J Pharmacol 1959; 14:48– 58. 20 Tallarida RJ, Murray RB. Manual of pharmacologic calcula- tions with computer programs. Springer-Verlag: New York, 1984, pp 30. 21 Schoeffter P, Godfraind T. Characterization of histamine- induced contraction in rat isolated aorta. Eur J Pharmacol 1991; 197: 193 – 200. Acknowledgements 22 Hill SJ et al. International Union of Pharmacology. XIII. Classification of histamine receptors. Am J Physiol 1997; 49: 253 – 278. The authors thank the Guadalajara and Madrid 23 Arrang JM, Garbarg M, Schwartz JC. Auto-inhibition of brain Municipal Slaughterhouse for kindly supplying histamine release by a novel class (H3) of histamine receptor. fresh tissue. This study was financed by grant from Nature 1983; 302: 832 – 837. 24 Ea-Kim L, Oudart N. A highly potent and selective H3 agonist DGES (PM98=0088) Ministerio de Educaciońy relaxes rabbit middle cerebral artery, in vitro. Eur J Pharmacol Ciencia, Spain. 1988; 150: 393 – 396.

International Journal of Impotence Research Contractile response of horse deep dorsal penile vein to histamine AC Mart´ınez et al 92 25 Mart´ınez AC et al. Histamine receptors in isolated bovine 27 Venugopalan CS et al. Biphasic responses of equine colonic oviductal arteries. Eur J Pharmacol 1997; 326: 163 – 173. vessel rings to vasoactive inﬂammatory mediators. J Auton 26 Gruetter CA, Lemke SM, Valentovic MA, Szarek JL. Evidence Pharmacol 1998; 18: 231 – 237. that histamine is involved as a mediator of endothelium- dependent contraction induced by A23187 in bovine intrapulmonary vein. Eur J Pharmacol 1994; 257: 275 – 283.

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