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) Ca²⁺-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 Ca²⁺-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.