Endothelin (ET), a novel vasoconstrictor peptide containing three isopeptides [ET-1, ET-2, and ET-3 (ETs)], has various biological effects including vasoconstriction, mitogenesis, and steroidogenesis. We examined the ET-1-like immunoreactivity level in porcine follicular fluid and culture medium of porcine granulosa cells by RIA. The ET level in the follicular fluid was 9.4-14.2 pg/ml. These levels were within 0.42 to 0.62-fold of the porcine plasma level (22.7 +/- 3.1 pg/ml) (mean +/- SE). ET was detected in the culture medium of granulosa cells with and without LH treatment at the concentration of 56 +/- 9.3 and 4.9 +/- 1.2 pg/10(6) cells.h, respectively. We also examined whether ETs affect the luteinization of granulosa cells. ETs inhibited the LH-stimulated progesterone and cAMP accumulation in cultured porcine granulosa cells in a dose-dependent manner with an EC50 of 5 x 10(-11) M. ET-1, ET-2, and ET-3 (5 x 10(-8)M inhibited progesterone accumulation by 62.3 +/- 1.8, 59.8 +/- 4.0, and 63.3 +/- 5.7% in 6-day cultures, respectively, and significant inhibition was observed within 24 h of culture. ET-1, ET-2, and ET-3 (5 x 10(-8) M) inhibited the LH-stimulated cAMP accumulation in granulosa cells by 54.8 +/- 2.3, 55.4 +/- 7.1, and 55.5 +/- 6.2%, respectively, whereas they did not affect basal cAMP levels. As well as progesterone accumulation, ETs partially inhibited LH-stimulated morphological transformation of granulosa cells. In this study, we demonstrated that ET exists in follicular fluid and in the culture medium of granulosa cells, and that ET inhibited LH-induced progesterone accumulation, morphological transformation, and cAMP accumulation in cultured porcine granulosa cells. These findings suggest that ET acts as a modulator of steroid metabolism in preovulatory follicles.