Special Transport Mechanisms in Gallbladder Transport Across Gallbladder • The main transport function of the gallbladder is the absorption of NaCI and water in near-isosmotic proportions • This involves apical membrane entry of Na+ and Cl- and basolateral membrane extrusion of both ions. • The Na+ and Cl- influxes across the apical membrane are via parallel, dependent Na+/H+ and Cl-/HC03 - exchangers • There is also NaCI or NaKCI cotransport. • Gallbladder epithelium also secretes K+ and H+ • Some mammalian species (e.g. the guinea pig) HC03 - is secreted instead of absorbed . • Basolateral membrane Na+ exit is mediated by the (Na+, K+) activated ATPase, and CI- extrusion appears to result from both conductive transport and electroneutral KC1 cotransport. Both cell membranes are K+ selective. • K+ channels originating from apical or basolateral membranes have been identified EFFECT OF cAMP • Elevating intracellular cAMP levels decreases the rate of fluid absorption by gallbladder epithelium and in some species can elicit net secretion.

• A number of agents, including prostaglandins, , vasoactive intestinal peptide (VIP), bradykinin, and vasopressin, elevate intracellular cAMP levels, which suggests that this mechanism is responsible for the effects of these agents on salt and water transport

• Maximal elevations of cAMP, reduce apical membrane Na+/H+ exchange in N ecturus gallbladder by about 50%

• cAMP inhibit H+ secretion in guinea pig gallbladder. EFFECT OF cAMP on CI-/HC03 - transport

• Increases in intracellular cAMP levels reduces CI-/HC03 - exchange by about 50%, when the Na+/H+ exchanger is blocked. • Effect of cAMP on Apical Membrane Cl-Conductance Increases in cAMP produce cell membrane depolarization. • In Necturus gallbladder, the cAMP-induced apical membrane CI- Conductance is insensitive to agents that block Cl- channels in other cells. • The cAMP-induced fall in Cl- is due to the combined effects of inhibition of • Cl-/HC03 - exchange and stimulation of CI- REGULATION OF BASOLATERAL MEMBRANE Na+ AND Cl- TRANSPORT

• Changes in pump activity occur in response to primary increases or decreases in the rate of Na+ entry, i.e. an elevation of Na+ entry is expected to stimulate the pump, and a reduction in Na+ entry would inhibit it • The mechanism that explain the relationship between apical Na + entry and basolateral Na + extrusion is the change in Na+ concentration elicited by alterations of apical entry. • Regulation of Basolateral Membrane Cl- Transport • In low-HC03 - media the basolateral membrane CI- transport is small, about 6% of the basolateral conductance. Incubating the tissues in media buffered with HC03 -/C02 increases conductance Regulation of K+

K+ channel, is largely inactive (open probability < 0.15) at the resting membrane voltage, and activates steeply with depolarization of the membrane patch. In Triturus and Necturus K+ channels are activated by membrane depolarization, or elevation of internal Ca2+ levels, In Necturus gallbladder, K+ channels account for no more than 20% of the resting apical membrane conductance, Reference

• Luis Reuss, Yoav Segal, and Guillermo Altenberg. REGULATION OF ION TRANSPORT ACROSS GALLBLADDER EPITHELIUM Annu. Rev. Phvsiol. 1991. 53:361-73 Copyright © h9 I by Annual Reviews Inc. Transport Mechanisms in Exocrine

• Exocrine glands are comprised of an and a with different cell types respectively. These glands are found in many organs within the body and demonstrate a large variety in the function of their secretions. Locations of Exocrine Glands • - eccrine sweat glands and sebaceous glands. Eccrine sweat glands are the most widespread sweat in the body and are present on nearly every external body surface. The sweat produced is clear with little to no oil, which in contrast to sebaceous glands, also found on the skin, which secrete the more oily substance sebum. • Salivary Glands- The salivary glands in the mouth are another example of exocrine glands and include the parotid glands, submandibular glands, and sublingual glands. While each gland has a unique mixture of serous and mucous cells which are involved in serous and secretions respectively • Stomach- The exocrine glands that are present in the stomach include: pyloric glands, cardiac glands, and fundic glands. These glands incorporate many different cell types including parietal cells, chief cells, and G cells. Together they regulate the gastric pH, release didestive enzymes and assist with absorption of necessary vitamins and minerals. • - The exocrine pancreas assists in food by releasing a secretion rich in bicarbonate, which helps to neutralize the acidic environment created in the stomach. The secretion also includes digestive enzymes. • Duodenum- These exocrine glands are submucosal and produce a mucous product that protects the duodenum from acid released from the stomach. The alkaline nature of the secretion also activates intestinal enzymes to assist with food breakdown and absorption. • Breast- Mammary glands produce milk rich in nutrients that also provides passive immunity to a baby’s immune system. Mechanisms of Exocrine secretion • The three mechanisms by which exocrine glands release their secretions include , , and . • Merocrine glands are the most common- merocrine gland secretions exit the cell via exocytosis. In this method of secretion, there is no cell damage. An example of merocrine secretion is the eccrine . • Apocrine glands, in contrast, form buds of the membrane which break off into the duct, losing part of the cellular membrane in the process. A well-known apocrine gland is the breastmilk-producing . • Holocrine, in which the cellular membrane ruptures to release its product into the duct. is an example of holocrine secretion. • Physiology, Exocrine Gland. S. Caleb Freeman; Ahmad Malik; Hajira BasiStatPearls [Internet].Treasure Island (FL): StatPearls Publishing; 2020 Jan-.