Direct Measurements of cAMP Effects on Membrane Conductance, Intracellular Ca2+ and pH in Molluscan Neurons

Abstract

We have investigated the effects of cAMP on membrane conductances, internal Ca2+ levels, and internal pH in 14 identifiable giant neurons of Archidoris monteryensis. Injecting cAMP in amounts which should have elevated the internal concentration by 50–200 μM (ignoring possible breakdown) had a strong excitatory effect on all the neurons tested, forcing them into repetitive firing or into a burst generating mode. Heavier doses resulted in a large, reversible depolarization. The membrane mechanism underlying the excitation was a long lasting conductance increase to Na+ which was induced specifically by cAMP. The pH changes which resulted from cAMP injection were not as stereotyped as the membrane conductance change. The most commonly observed response was a transient acidification which persisted for 10 to 20 minutes following an injection. In some neurons, however, a transient alkalinization occurred. Both responses were within the range of.05 to.1 pH unit change and were graded with the size of the injection. Addition of the phosphodiesterase inhibitor IBMX to the bath before injection of cAMP caused a significant prolongation of both the current and pH responses. Multiwavelength absorbance measurements of the indicator dye, arsenazo III, were used to probe for changes induced by cAMP in resting [Ca2+] or in the characteristics of Ca influx or regulation during electrical activity. We were unable to detect significant changes in any of the above parameters.