MEMBRANE CURRENTS OF PORCINE GRANULOSA-CELLS IN PRIMARY CULTURE - CHARACTERIZATION AND EFFECTS OF LUTEINIZING-HORMONE

In ovarian granulosa cells, LH depolarizes the membrane potential and induces steroid production. The membrane currents of porcine granulosa cells in primary culture were studied by means of the whole-cell conf iguration of the patch-clamp technique to investigate whether the ioni c channels are involved in the effects of LH. We identified and charac terized two types of K+ currents-a transient outward current (I(to)) a nd a delayed rectifier K+ current (I(K))-and one Ca2+ current I(to) an d I(K) were voltage- and calcium-dependent. Both currents were blocked by 4-aminopyridine, a K+ channel blocker, but only I(K) was sensitive to tetraethylammonium, another K+ channel blocker. I(Ca) was inactiva ted within 50 ms of the test pulse. Nifedipine and verapamil, L-type C A2+ channel blockers, did not suppress I(Ca) even at a concentration o f 10 muM. Tetramethrin (1 muM), a T-type Ca2+ channel blocker, decreas ed I(Ca) by 38.4 +/- 5.6% (n = 3). These findings suggest that the cur rent may be a T-type Ca2+ current. LH and dibutyryl cAMP, potent stimu lants of steroid production, attenuated I(to) only by 13.9 +/- 1.8% (n = 7) and 21.0 +/- 1.5% (n = 4), respectively. However, they did not a ffect 1(K) and I(Ca). These results indicated that LH does not modulat e CA2+ current directly, but it slightly decreased I(to) through cAMP elevation. The LH-induced inhibition of I(to) may be involved in the d epolarizing effect of LH and may play some role in steroid secretion o r other functions in granulosa cells.