Modification of ATP-dependent Ca2+ transport in rat parotid basolateral membranes during aging

Previous experiments have shown that senescent rat parotid acinar cells display marked reductions in Ca2+ release following alpha 1-adrenoreceptor stimulation. We report here, that in this naturally occurring perturbation of exocrine secretion, ATP-dependent Ca2+ transport in the parotid basolateral plasma membrane, the principal Ca2+ extrusion pathway in the parotid, is also modified. ATP-dependent Ca2+ transport in membrane vesicles isolated from senescent rats (approx. 24 months) is decreased approximately 30-50% as compared to that in vesicles isolated from younger rats (approx. 4 months). This alteration in Ca2+ pump activity is not due to (i) non-specific effects of vesicle preparation in the two animal groups, (ii) increased leakiness to Ca2+, or (iii) any apparent alteration in permeability of the membrane to K+ and Cl-. Kinetic studies demonstrate that the ATP-dependent Ca2+ transport activity in vesicles from senescent rats has similar maximal velocity to that of vesicles from young adult rats (27 vs. 31 nmol Ca2+/mg protein per min), however, it exhibits an approximately 50% increase in Km for Ca2+ (91 nM vs. 60 nM). Cytosolic free Ca2+, measured by Quin 2 fluorescence, in parotid acini following alpha 1-adrenoreceptor stimulation was much less elevated in preparations from senescent rats. These results may account, at least in part, for the previously reported physiological alteration in Ca2+ efflux seen in senescent rat parotid cells.