Properties of an LH-stimulable adenylate cyclase in plasma membranes of interstitial cells from rat testis.

Abstract

We studied the effect of guanyl nucleotides, divalent cations and luteinizing hormone (LH) on the regulation of adenylate cyclase (AC) in partially purified plasma membranes obtained from isolated interstitial cells of the rat testis. AC was activated to different degrees by guanosine triphosphate (GTP) and GMP-P(NH)P; the latter was about 10 times more active than the former. Enzyme activation by GTP was biphasic; the nucleotide was rapidly hydrolysed by membrane preparation. Activation by GMP-P(NH)P was hysteretic, requiring about 20-30 min to reach steady state; this lag-time was not dependent on nucleotide concentration. GDP beta S did not stimulate AC activity. Delayed addition of GDP beta S to a GMP-P(NH)P-stimulated enzyme at 17 min resulted in a drop of AC activity although the activity 40 min later was higher than that obtained by mixing both nucleotides at the time the reaction was initiated. This result was incompatible with the formation of a truly irreversible, active form of the enzyme in the presence of GMP-P(NH)P. The effect of LH on AC depended on guanine nucleotides and Mg2+. LH and GMP-P(NH)P acted synergically. Dose-response curves showed that apparent LH affinity was not modified by the presence of GMP-P(NH)P. LH accelerated the slow rate of activation of GMP-P(NH)P. The stimulation of AC by LH was closely dependent on Mg2+ concentrations; LH diminished the apparent Mg2+ requirement. Plasma membranes from rat testicular interstitial cells are an excellent model for the study of AC regulation by LH.