The effects of interleukin on testicular steroidogenesis have been studied in several laboratories, most often by using cultured rat Leydig cells. Several reports have indicated that interleukin-1 beta (IL-1 beta), but not interleukin-1 alpha (IL-1 alpha), exert a potent effect on gonadotropin action in rat Leydig cells. By using cultured porcine Leydig cells as a model, we found that IL-1 alpha (and to a lesser extent IL-1 beta), contrary to previous reports, is a potent inhibitor of LH/hCG steroidogenic action; and we further localized the steroidogenic biochemical step(s) affected by IL-1 alpha. IL-1 alpha inhibited hCG-induced testosterone secretion (about 67%) in a dose- and time-dependent manner. Half maximal and maximal effects were obtained with 4 U/ml (approximately 0.4 ng/ml, 0.3 x 10(-10) M) and 20 U/ml (approximately 2 ng/ml, 1.4 x 10(-10) M) of IL-1 alpha, respectively. The inhibitory effect of IL-1 alpha on gonadotropin action was detected at 6 h and was maximal after 24 h of treatment with the cytokine. The IL-1 alpha inhibitory effect was more potent than that of IL-1 beta: the maximal inhibitory effect of IL-1 beta was obtained with 400 U/ml. Subsequent investigations indicated that IL-1 alpha inhibited different biochemical steps involved in gonadotropin-induced testicular steroidogenesis. In this context, although IL-1 alpha appears to inhibit Leydig cell membrane functions (through a decrease in LH/hCG binding and gonadotropin-induced cAMP production), the antigonadotropin action of the cytokine is probably exerted predominantly at a step(s) located beyond cAMP formation.(ABSTRACT TRUNCATED AT 250 WORDS)