Calcium regulation of the soluble adenylyl cyclase expressed in mammalian spermatozoa

Abstract

In mammals, Ca 2+and\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}ions play a critical role in the regulation of sperm function, most likely by regulation of cAMP levels. Mammalian germ cells contain a soluble adenylyl cyclase (sAC) with properties distinct from the well characterized membrane-bound enzymes Here we investigated whether the cyclase expressed in mature spermatozoa has the properties of sAC and whether it is regulated by Ca 2+. In addition to an\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}-dependent activation, the cyclase endogenous to human spermatozoa is stimulated 2- to 3-fold by Ca 2+in a concentration-dependent manner (EC 50≈ 400 nM). In a similar fashion, Ca 2+activates the recombinant rat and human full-length sAC with similar EC 50 values. The Ca 2+stimulation was also observed when sAC was activated with\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}, was independent of calmodulin, and was associated with an increase in Vmax without changes in Km for ATP-Mg 2+. An increase in intracellular Ca 2+by ionophore or by a muscarinic cholinergic receptor agonist increases cAMP in cells transfected with FL-hsAC, but not in mock-transfected cells. Similarly, both Ca 2+and\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}stimulate cAMP accumulation in human spermatozoa. These findings provide evidence that human spermatozoa express a cyclase with the properties of sAC and that Ca 2+can substitute for\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{HCO}}_{3}^{-}\end{equation*}\end{document}in the stimulation of this enzyme, underscoring an important role for sAC in the control of sperm functions.