1. Whole‐cell voltage‐clamp techniques were used to study the voltage‐dependent membrane conductances in parasympathetic neurones enzymatically isolated from the interatrial septum of bull‐frog heart and maintained in short‐term (1‐10 day) tissue culture. 2. The resting potential of the isolated neurones averaged ‐55.4 +/‐ 1.1 mV (+/‐ S.E.M., n = 11). Action potentials evoked in the isolated cells by brief (1‐2 ms) current injections were similar to those recorded from neurones in the ‘intact’ septum. The amplitude of action potentials of isolated neurones averaged about 113 mV, with a peak depolarization of +32.8 +/‐ 2.8 mV and after‐hyperpolarization of ‐80.0 +/‐ 2.8 mV. 3. The pattern of membrane currents recorded using voltage clamp with ‘normal’ external (containing 110 mM‐Na+) and internal (110 mM‐K+) solutions consisted of a rapidly activating and inactivating inward current followed by a slower, sustained outward current. 4. The inward components of current were isolated by using an internal solution in which Cs+ and TEA+ (tetrathylammonium) ions replaced K+. Depolarizations from holding potentials of ‐50 to ‐70 mV produced inward currents which had an initial transient phase followed by a maintained, or very slowly inactivating, component. The current‐voltage relation for the initial transient phase reached a peak at membrane potentials near 0 mV, while the maintained phase, measured, for example, at the end of 50 ms voltage‐clamp steps, had its peak near +10 mV. 5. The transient component of inward current was carried primarily by Na+ ions, as replacement of Na+ by TEA+ in the external solution abolished the transient. This current was thus identified as a voltage‐dependent Na+ current, INa. The maintained component was greatly attenuated by removing 80‐90% of the external Ca2+ ions, and it was abolished by divalent cations such as Cd2+ (0.2‐0.4 mM), Ni2+ (0.5 mM) and La3+ (10‐100 microM). This maintained component was thus a voltage‐dependent calcium current, ICa. 6. About 80% of INa recorded in the presence of low (0.2‐0.5 mM) external Ca2+ and 2 microM‐LaCl3 was blocked by tetrodotoxin (TTX) with an apparent Kd of about 8 nM. The remaining 20% of INa was resistant to block by 2‐10 microM‐TTX. However, the ‘TTX‐resistant’ component of INa was blocked by Cd2+ (0.2‐0.4 mM). 7. The voltage‐dependent calcium current, ICa, measured in saline in which Na+ was replaced by N‐methyl‐D‐glucamine, activated near ‐40 mV and reached a peak near +10 to +15 mV.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society
