Two types of high-threshold calcium currents inhibited by omega-conotoxin in nerve terminals of rat neurohypophysis

Abstract

1. The neurohypophysis comprises the nerve terminals of hypothalamic neurosecretory cells, which contain arginine vasopressin (AVP) and oxytocin. The secretory terminals of rat neurohypophyses were acutely dissociated. The macroscopic calcium currents (ICa) of these isolated peptidergic terminals were studied using 'whole-cell' patch-clamp recording techniques. 2. There are two types ('Nt' (where the subscript 't' denotes terminal) and 'L') of high-threshold voltage-activated ICa in the terminals, which can be distinguished by holding at different potentials i.e. -90 and -50 mV. Replacement of Ca2+ in the bathing solution by Ba2+ increased the amplitude of ICa, primarily due to an increase in the L-type component. Both inward currents were eliminated by adding 50 microM-Cd2+ or when in a Ca(2+)-free bathing solution. 3. omega-Conotoxin GVIA (omega-CgTx) has been widely used as a Ca2+ channel blocker. However, whether this toxin can discriminate between different types of Ca2+ channels is still a subject of controversy. We applied omega-CgTx over a wide range of concentrations (0.01-2 microM) to examine its effects on both Nt- and L-type ICa in these terminals. At a concentration of 30 nM, omega-CgTx selectively reduced, by 48%, the amplitude of Nt-type ICa. In contrast, a higher concentration (300 nM) of omega-CgTx was necessary to inhibit the L-type ICa. 4. omega-CgTx inhibited both Nt- and L-type ICa in a dose-dependent manner, and the half-maximum inhibition (IC50) of the ICa by the toxin was 50 and 513 nM, respectively, which was approximately a tenfold difference. The reduction in both types of currents did not result from any shift in their current-voltage or steady-state inactivation relationships. 5. In contrast, omega-CgTx, at a concentration of 300 nM, had no effect on the tetrodotoxin-sensitive sodium current (INa) of the isolated peptidergic nerve terminals. Furthermore, omega-CgTx did not reduce the long-lasting, non-inactivating ICa in the isolated non-neuronal secretory cells of the pars intermedia (PI) (intermediate lobe of the pituitary). 6. Our studies suggest that omega-CgTx might exert specific blocking effects on both Nt- and L-type Ca2+ channels, but that in the isolated peptidergic nerve terminals, the Nt-type component is more susceptible to this toxin.