The effects of a novel membrane-penetrable modulator, 2APB (2-aminoethoxy diphenyl borate), on Ins(1,4,5)P3-induced Ca2+ release were examined. 2APB inhibited Ins(1,4,5)P3-induced Ca2+ release from rat cerebellar microsomal preparations without affecting [3H]Ins(1,4,5)P3 binding to its receptor. The IC50 value (concentration producing 50% inhibition) of 2APB for inhibition of Ins(1,4,5)P3 (100 nM) induced Ca2+ release was 42 microM. Further increase in the concentration of 2APB (more than 90 microM) caused a gradual release of Ca2+ from cerebellar microsomal preparations. Addition of 2APB to the extracellular environment inhibited the cytosolic Ca2+ ([Ca2+]c) rise in intact cells such as human platelets and neutrophils stimulated by thromboxane-mimetic STA2 or thrombin, and leukotriene B4 (LTB4) or formyl-methionine-leucine-phenylalanine (FMLP), respectively. 2APB inhibited the contraction of thoracic aorta isolated from rabbits induced by angiotensin II (AII), STA2, and norepinephrine in a non-competitive manner, but showed no effect on the contraction of potassium-depolarized muscle. 2APB had no effect on the Ca2+ release from the ryanodine-sensitive Ca2+ store prepared from rat leg skeletal muscle and heart. Although the specificity of 2APB with respect to the intracellular signaling system was not fully established, 2APB is the first candidate for a membrane-penetrable modulator of Ins(1,4,5)P3 receptor, and it should be a useful tool to investigate the physiological role of the Ins(1,4,5)P3 receptor in various cells.

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SUMMARY1. Intracellular recordings were made from chromaffin cells isolated from adrenal medullae of gerbils to examine the effects, on membrane potential, of changes in the ionic environment that are known, from other experiments, to influence the rate of catecholamine secretion.2. Depolarization in response to acetylcholine fell linearly with the logarithm of the extracellular sodium concentration over the range 154-3 mM and reached a value, in sodium-free medium, of about 30 % of the control value.3. The depolarizing effect of acetylcholine in sodium-free media increased linearly with the logarithm of the extracellular calcium concentration over the range 1-117 mM.It is concluded that depolarization in response to acetylcholine involves inward movement of both sodium and calcium ions.4. Depolarization was also observed in response to the secretagogues, excess potassium and barium, both in sodium-rich and sodium-free media. The effect of barium was antagonized by calcium, and it is suggested that these two cations interact at the level of the plasma membrane.5. Depolarization does not appear to be tightly coupled to secretion, for acetylcholine or excess potassium still depolarized the chromaffin cells when the environment was calcium-free or contained an excess of magnesium, conditions that inhibit secretion. Furthermore, although acetylcholine had some depolarizing effect in sodium-free media, the level to which the membrane potential fell was not below the control 'resting'

SUMMARY1. Transmembrane potential, effective membraneresistance, and amylase output were recorded from acinar cells of rat pancreas perfused in vitro.2. Both pancreozymin and acetylcholine hyperpolarized the acinar cells, increased effective membrane resistance, and augmented amylase output.3. The omission of calcium from the perfusion medium increased effective membrane resistance and potential, and abolished the increase in amylase output in response to the drugs. 4. A quantitative relation was found between the amount of amylase released by pancreozymin and the concentration of calcium in the perfusion medium at values below the normal 2*5 mm calcium. Excess magnesium did not inhibit the increase in amylase output in response to the drug.5. It is concluded that the release of amylase from the pancreas depends on the entry of calcium into the acinar cells. The entry of calcium seems to be mediated by a carrier in the membrane and may be considered as a 'facilitated diffusion'.6. The electrophysiological findings taken together with morphological evidence provided by scanning electron microscopy favour the view that pancreatic zymogens are released from the granules in the acinar cells to the lumen by the process known as exocytosis.

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Photodynamic agents, due to their photon‐dependent selective activation, can selectively activate a number of physiological processes and may directly modulate signal transduction in a number of cells including pancreatic acinar cells. Activation of the photodynamic agent sulphonated aluminium phthalocyanine (SALPC) triggered recurrent cytosolic calcium ([Ca2+]i) spiking in pancreatic acinar cells. The photodynamically triggered calcium spiking could be blocked by phosphatidylinositol‐specific phospholipase C (PI‐PLC) inhibitor U73122, but not by phosphatidylcholine‐specific phospholipase C inhibitor D609. Removal of extracellular Ca2+ abolished spiking, as did 2‐aminoethoxydiphenylborate (2‐APB), an inhibitory modulator of IP3‐mediated Ca2+ release from intracellular stores. These data suggest that SALPC photodynamic action may permanently fix PI‐PLC in an active conformation, and this produced recurrent [Ca2+]i spiking.

A B S T R A C T Using an isolated perfused rat pancreas preparation, the interrelationship between the endocrine and exocrine portions of the pancreas were studied. Addition of exogenous rat insulin (1-20 mU/ml) to the perfusing solution potentiated the action of cholecystokinin (CCK) (1 mU/ml) to increase both pancreatic juice flow and the release of the enzyme, amnylase. Raising the glucose concentration in the perfusing solution from 2.5 to 17.5 mM both increased endogenous insulin release and potentiated the CCKinduced exocrine secretory response. Two lines of evidence indicated that this effect of glucose on the exocrine pancreas was mediated by endogenous insulin release. First, the addition ofcomparable amounts of xylose or galactose to the perfusion medium neither released insulin nor potentiated the CCK-induced response. Second, epinephrine blocked the effect of high glucose on both insulin release and potentiation of CCK action. Epinephrine alone did not affect the action of CCK. The magnitude of the exocrine response induced by high glucose was comparable to that of 2.5 mU/ml exogenous insulin. It seems possible that pancreatic acinar cells can be exposed to insulin levels of this magnitude in situ.

SUIMMARY1. A method has been devised for isolating adrenal chromaffin cells (from gerbils) and maintaining them in vitro. Transmembrane potentials of these cells were recorded with intracellular micro-electrodes.2. Acetylcholine depolarized the chromaffin cells and so did various other substances known to evoke catecholamine secretion: nicotine, pilocarpine, histamine, 5-hydroxytryptamine, angiotensin, and bradykinin.3. The depolarizing effect of acetylcholine was partially antagonized by hexamethonium and was blocked completely by hexamethonium in combination with atropine.4. Hexamethonium alone completely blocked the response to nicotine; and atropine alone abolished the response to pilocarpine. Thus both nicotinic and muscarinic receptors are present in gerbil chromaffin cells.5. The experiments demonstrate that the various secretagogues and antagonists tested act on the plasma membrane of the chromaffin cell and raise the question whether depolarization may be an important event in stimulus-secretion coupling.