Voltage-gated calcium currents in the magnocellular neurosecretory cells of the rat supraoptic nucleus.

1. Whole-cell patch-clamp techniques were used to analyse voltage-dependent calcium currents in acutely isolated somata of magnocellular neurosecretory cells (MNCs) from the supraoptic nucleus of the hypothalamus of adult rats. Currents were characterized on the basis of their rates of inactivation and their sensitivity to a series of calcium channel blocking agents. 2. Curve fitting analysis of series of long lasting depolarizing voltage steps from a holding potential of -80 mV revealed three current components with different voltage dependences and rates of inactivation (n = 36). These include a low threshold (-60 mV), rapidly inactivating (tau = 42 +/- 3 ms at -10 mV) component, a high threshold (-30 mV), slowly inactivating (tau = 1790 +/- 70 ms) component and a component with an intermediate threshold (-50 mV) and rate of inactivation (tau = 187 +/- 15 ms). There is also a non-inactivating portion of evoked calcium current with a threshold of -50 mV. 3. Based on its voltage dependence, rate of inactivation, greater sensitivity to the divalent cation nickel than to cadmium and insensitivity to omega-conotoxin GVIA (omega-CgTX), the low threshold current appears to be a T-type calcium current. The rate of inactivation, voltage dependence, and sensitivity to omega-CgTX of the slowly inactivating component suggests that it is an N-type current. The characteristics of the intermediate component do not correspond to any identified calcium current type. 4. Portions of the non-inactivating calcium current are sensitive to nifedipine (23 +/- 2% of the total non-inactivating current at -10 mV; n = 10), suggesting the presence of L-type currents, omega-agatoxin-IVA (omega-Aga-IVA; 20 +/- 6% of total; n = 11), suggesting the presence of P-type channels, and omega-CgTX (39 +/- 3% of total; n = 19), suggesting the presence of a non-inactivating N-type current. The non-inactivating component at low potentials (> or = -50 mV) was selectively blocked by nifedipine, suggesting the presence of a novel, low threshold L-type current. 5. We conclude that MNC soma express T-, N-, L-, and P-type calcium currents, as well as a novel low threshold nifedipine-sensitive current and an unidentified inactivating component. This complement of currents is different from that seen in the terminals of these cells, suggesting a spatial and functional segregation of calcium current types in MNCs.