Characterization and modulation of Na+ and Ca2+ currents underlying the action potential in bag cells of two species of Aplysia.

Abstract

The neurosecretory bag cells of Aplysia produce long trains of action potentials (afterdischarge) to release hormones important to egg laying. These ionic currents are modulated by second messengers. Modulation of excitability in bag cells is incompletely understood partly because the currents that are modulated have not been fully characterized. Whole-cell voltage-clamp experiments were executed in cultured cells from sexually mature A. californica and A. brasiliana to characterize the inward voltage-gated currents for Na+ and Ca2+. These species had similar Na+ and Ca2+ current characteristics. The Na+ currents activated at voltages less negative than -30 mV and peaked at between +10 and +20 mV in artificial sea water. The time course and pharmacology of bag cell Na+ currents were similar to those of fast Na+ current in other excitable cells. Na+ currents were abolished in Na(+)-free extracellular solution and were not inhibited by Cd2+. The KD for inhibition by tetrodotoxin was 2.6 nmol l-1. The Na+ current was relatively insensitive to depolarized holding potentials (Vh), maintaining approximately 65% of peak current amplitude throughout the activation range at Vh = -30 mV. In experiments using a 1 s depolarized Vh prior to a test pulse, the half-inactivation voltage (V1/2) was -21 mV. The time constant of recovery from steady-state activation was 2.9 ms at Vh = -70 mV and 6.8 ms at Vh = -30 mV. The Ca2+ currents activated near -10 mV and peaked at approximately +20 mV with 11 mmol l-1 Ba2+ as the charge carrier. The pharmacology and V1/2 of bag cell Ca2+ current were similar to those of L-type Ca2+ currents. In extracellular solution without Na+, but containing Ba2+, Cs+ and tetraethylammonium, the Ca2+ current was inhibited by 25-100% by nifedipine (10 mumol l-1), mean 42%, and was unaffected in the majority of cells by omega-conotoxin (10 mumol l-1). The Ca2+ current was insensitive to Ni2+ (100 mumol l-1), but was abolished by 100 mumol l-1 Cd2+. Like the Na+ current, the Ca2+ current was relatively insensitive to depolarized Vh, maintaining more than 80% of peak current amplitude throughout the activation range at Vh = -40 mV. With a 1 s depolarized Vh prior to a test pulse, the V1/2 was -30 mV.(ABSTRACT TRUNCATED AT 400 WORDS)