Effects of various cations on the slow K+ conductance increases induced by carbachol, histamine and dopamine in Aplysia neurones.

A study was made of the effects of various cations other than K+ on three K+ conductance increases induced by carbachol, histamine and dopamine in an identified group of Aplysia neurones: the 'A' neurones of the cerebral ganglion. The 3 responses were sensitive to alterations of both the extracellular and the intracellular concentrations of Na+ and Ca2+. In particular, they could be reduced markedly by: (a) lowering [Na]0 (replacing NaCl by either Tris-HCl, glucosamine chloride, MgCl2 or sucrose); (b) increasing [Na]i (by intracellular injection of Na+, or by blockade of the Na+-K+ pump); (c) increasing the extracellular divalent cation concentration; or (d) increasing [Ca]i4. Some of the effects of Na+ and divalent cations appear to occur on reaction steps common to the three K+ responses, while others probably imply reaction steps specific to one of the systems, since they differ according to the agonist used. The sensitivity to Na+ and Ca2+ of slow inhibitory responses due entirely to an increase in K+ conductance must be taken into account in the interpretation of some slow hyperpolarizing responses previously assumed to involve changes in Na+ conductance.