Properties of Na+ currents in putative submandibular and cardiac sympathetic postganglionic neurones.

This study was performed to compare the kinetic properties of Na+ currents in putative salivary and cardiac postganglionic sympathetic neurones isolated from the superior cervical and stellate ganglia, respectively. Neurones were labelled with a fluorescent tracer-Fast Blue, injected into the submandibular gland (in the case of salivary neurones) and into the pericardial cavity or left ventricular wall (in the case of cardiac neurones). Voltage-dependent Na+ current was then isolated and recorded from labelled cells. The major findings of this study were: (1) Peak Na+ current was larger in salivary than in cardiac neurones (5.7 nA vs. 2.4 nA; for 30 mM Na+ in extra- and 15 mM in the intracellular solution). (2) The somata of salivary neurones were twice as large as those of cardiac neurones, as indicated by the values of their membrane capacitance (36 pF vs. 18 pF). (3) There was a greater Na+ current density (169 pA/pF vs. 128 pA/pF) in salivary than in cardiac neurones. (4) Recovery from inactivation was faster in salivary neurones with 90% recovery time being 93 ms for salivary and 144 ms in cardiac neurones. (5) Half-activation times were voltage-dependent and consistently longer for salivary than for cardiac neurones. (6) Remaining parameters, such as current threshold, maximum current voltage and kinetics of steady-state inactivation did not significantly differ in salivary compared to cardiac neurones.