A calcium‐induced calcium‐release mechanism supports Luteinizing Hormone (LH) induced testosterone secretion in mice Leydig cells

Abstract

Binding of LH to a G Protein Coupled Receptor in the plasma membrane, results in a primary increase in cAMP and in the intracellular calcium concentration ([Ca2+]i in Leydig cells. Immunocytochemical labeling with specific antibodies show the presence of three distinct isoforms of both ryanodine receptors (RYRs) and inositol 1,4,5‐trisphosphate receptors (IP3Rs). Measurements of intracellular calcium activity, using the fluorescent calcium sensitive dye Fluo3 and confocal microscopy, demonstrate that these receptors are involved in a calcium induced calcium release mechanism (CICR), responsible for the amplification of the initial Ca2+ influx through plasma membrane T‐type calcium channels (CaV3), induced by LH. The RYRs and IP3Rs are functional, as judged both from their activation by caffeine and IP3 and block by ryanodine and 2‐APB, respectively. Nevertheless, the intracellular global calcium changes resulting from treatment of the cells with LH are readily blocked 100 μM ryanodine but not by 2‐APB or xestospongin C. Besides this, blockage of the ryanodine receptor inhibits both the hormone‐induced [Ca2+]i transients and steroid secretion by Leydig cells. These results not only broaden the understanding of the role played by calcium in Leydig cells but also show, for the first time, that ryanodine receptors have a crucial role in determining secretion of testosterone by the testis.Financial Support: FAPESP, CNPq