The neoglycoprotein mannosebovine serum albumin, but not progesterone, activates T-type calcium channels in human spermatozoa

Abstract

The neoglycoproteins α-D-mannose–bovine serum albumin (mannose–BSA) and N-acetyl-α-D-glucosamine–BSA (glucNAc–BSA) were shown to rapidly increase intracellular free calcium ([Ca²⁺]_i) in human spermatozoa. The increase in [Ca²⁺]_i induced by these neoglycoproteins accounts for the known ability of these compounds to induce the acrosome reaction in human spermatozoa. Our data support the hypothesis that mannose–BSA, but not progesterone, activates T-type Ca²⁺ channels in human spermatozoa for the following reasons: (i) the capacity of mannose–BSA to increase [Ca²⁺]_i was inhibited by the specific T-type Ca²⁺ channel blocker mibefradil (IC₅₀ = 10^–6 mol/l) while progesterone was not inhibited by 10^–5 M mibefradil; (ii) the effect of mannose–BSA to elevate [Ca²⁺]_i was inhibited more potently by Ni²⁺ (IC₅₀ = 0.1 mmol/l) than Cd²⁺ (IC₅₀ = 0.5 mmol/l), whereas the effect of progesterone to elevate [Ca²⁺]_i was inhibited equally by Ni²⁺ and Cd²⁺ (IC₅₀ = 0.25 mmol/l); (iii) the effects of mannose–BSA and progesterone to increase [Ca²⁺]_i were greater than additive. These data support the idea that mannose–BSA and progesterone were activating distinct Ca²⁺ channels, one of which was a T-type Ca²⁺ channel activated by mannose–BSA whereas the Ca²⁺ channel that was activated by progesterone has yet to be defined at the molecular level.