The mechanism of bicarbonate’s action in increasing zona binding capability remains to be elucidated. The relatively slow time course of the binding change suggests that it results from a multistep process. Given the accelerating effects of caffeine or IBMX in the presence of bicarbonate, it could be that bicarbonate acts through direct stimulation of adenylyl cyclase ; see also ), but how the putative rise in cAMP might bring about the bicarbonate-dependent changes in membrane lipid organization or dissociation of surface components is at present quite unknown. Similarly, the site of action of Ca2+ is uncertain. buy asthma inhalers
Sperm that had been incubated for 2 h in the standard Tyrode’s medium were able to bind FITC-sZP binding even after dilution in BTS (which contains EDTA); thus FITC-sZP binding itself is not dependent on calcium. Calcium’s role must therefore be in the development of the binding ability. Since this development appears to parallel changes in the sperm’s surface coating, calcium may well act externally. However, development of the binding sites could also involve an intracellular process dependent on calcium that is brought into play after a membrane reorganization that enhances a calcium influx. EGTA would prohibit such an influx, or could even lead to a decrease in cytoplasmatic calcium levels .