Ca\u3csup\u3e2+\u3c/sup\u3e influx, but not Ca\u3csup\u3e2+\u3c/sup\u3e release from internal stores, is required for the PACAP-induced increase in excitability in guinea pig intracardiac neurons

Mechanisms modulating the pituitary adenylate cyclase activating polypeptide (PACAP)-induced increase in excitability have been studied using dissociated guinea pig intrinsic cardiac neurons and intact ganglion preparations. Measurements of intracellular calcium (Ca ) with the fluorescent Ca indicator dye fluo-3 indicated that neither PACAP nor vasoactive intestinal polypeptide (VIP) at either 100 nM or 1 μM produced a discernible elevation of intracellular Ca in dissociated intracardiac neurons. For neurons in ganglion whole mount preparations kept in control bath solution, local application of PACAP significantly increased excitability, as indicated by the number of action potentials generated by long depolarizing current pulses. However, in a Ca -deficient solution in which external Ca was replaced by Mg or when cells were bathed in control solution containing 200 μM Cd , PACAP did not enhance action potential firing. In contrast, in a Ca -deficient solution with Ca replaced by strontium (Sr ), PACAP increased excitability. PACAP increased excitability in cells treated with a combination of 20 μM ryanodine and 10 mM caffeine to interrupt release of Ca from internal stores. Experiments using fluo-3 showed that ryanodine/caffeine pretreatment eliminated subsequent caffeine-induced Ca release from intracellular stores, whereas exposure to the Ca -deficient solution did not. In dissociated intracardiac neurons voltage clamped with the perforated patch recording technique, 100 nM PACAP decreased the voltage-dependent barium current (I ). These results show that, in the guinea pig intracardiac neurons, the PACAP-induced increase in excitability apparently requires Ca influx through Cd -sensitive calcium permeable channels other than voltage-dependent Ca channels, but not Ca release from internal stores. Copyright © 2006 The American Physiological Society. 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ 2+ B