Intracellular Ca2+ homeostasis in rat round spermatids.

Intracellular calcium, [Ca2+]i, can regulate meiotic progression of mammalian oocytes. However, the role of [Ca2+]i in the regulation of the spermatogenic process and its cellular homeostatic mechanisms in spermatogenic cells has not been elucidated. Using intracellular fluorescent probes for Ca2+ and immunodetection of plasma membrane (PM) Ca(2+)-ATPases, we report that: a) rat round spermatids maintain [Ca2+]i levels of 60 +/- 5 nM (SEM), as estimated with fluo-3 in single cells or fura-2 in cells in suspension; b) these cells regulate [Ca2+]i by actively extruding it using a PM Ca(2+)-ATPase; c) rat spermatids also actively transport Ca2+ by sarco-endoplasmic reticulum type ATPases (SERCA); d) rat spermatids possess non-mitochondrial intracellular Ca2+i stores insensitive to thapsigargin but releasable by ionomycin; and e) rat spermatids do not activate Ca2+ entry mechanisms by the release of Ca2+ from SERCA-regulated stores. These results demonstrate that rat round spermatids can generate modulated intracellular Ca2+ signals upon activation of Ca2+ channels or Ca2+ release from intracellular stores.