Localization and functional modification of L-type voltage-gated calcium channels in equine spermatozoa from fresh and frozen semen

It is well known that insemination of cryopreserved semen always results in lower fertility when compared with fresh semen, but there is an increased interest and demand for frozen equine semen by the major breeder associations because of the utility arising from semen already “on hand” at breeding time. In this article, we report that equine sperm cells express L-type voltage-gated calcium channels; their localization is restricted to sperm neck and to the principal piece of the tail in both fresh and frozen-thawed spermatozoa. We also studied the causes of cryoinjury at the membrane level focusing on the function of L-type calcium channels. We report that in cryopreserved spermatozoa the mean basal value of [Ca2+]i is higher than that of spermatozoa from fresh semen (447.130 vs. 288.3 nM; P < 0.001) and L-type channels function differently in response to their agonist and antagonist in relation to semen condition (fresh or frozen-thawed). We found that on addition of agonist to the culture medium, the increase in intracellular calcium concentrations ([Ca2+]i) was greater in frozen semen than in fresh semen (Δ[Ca2+]i = 124.59 vs. 16.04 nM; P < 0.001), whereas after the addition of antagonist the decrease in [Ca2+]i was lower in frozen semen than in fresh semen (Δ[Ca2+]i = 32.5 vs. 82.5 nM; P < 0.001). In this article, we also discuss the impact of cryopreservation on sperm physiology.