Propranolol stimulates histone phosphorylation by a putative PK-C in partially purified homogenate of rat testicular interstitial cells. A possible mechanism for increased testosterone secretion by propranolol

The beta-adrenoceptor blocker propranolol stimulated testosterone secretion by rat testicular interstitial cells (Leydig cell-enriched preparation) in vitro at concentrations ranging from 10(-5) M to 10(-4) M. Treatment of th ese cells with H7 (20 mu M), an inhibitor of protein kinase C, reduced the stimulatory effect of L-propranolol on testosterone secretion by about 5-fo ld. At concentrations ranging from 31.25 mu M to 1000 mu M, L-propranolol r educed [H-3]phorbol 12,13-dibutyrate binding (IC50 = 75 mu M) to rat testic ular interstitial cells. At similar concentrations, L-propranolol displaced the binding of [H-3]phorbol 12,13-dibutyrate to the homogenate of these ce lls by only 5%. These findings suggest: that the effect of L-propranolol on [H-3]phorbol 12,13-dibutyrate binding could be indirect, possibly by incre asing the concentration of a chemical mediator interacting with the regulat ory domain of protein kinase C. At even lower concentrations (10(-9) M to 1 0(-7) M), propranolol added directly to the reaction mixture with protein k inase C partially purified from rat testicular interstitial cells increases the phosphorylation of histone. This phosphorylation was comparable to tha t obtained with (25 mu g/ml) phosphatidylserine. The D- and L-stereoisomers of propranolol were equally active. A complete reversal of this propranolo l effect on histone phosphorylation was achieved with (20 mu M) H-7. In the absence of Ca2+, propranolol was not able to phosphorylate the histone. Ta ken together, these results suggest that protein kinase C could be the puta tive kinase involved in this reaction and that its activation by propranolo l may be due to interaction of the drug with the regulatory domain of the e nzyme at a site differing from the site of interaction with phorbol 12,13-d ibutyrate. The ability of propranolol to activate the putative protein kina se C could be related to its stimulatory effect on testosterone secretion b y Leydig cells.