Active Ca2+ transport systems in basolateral membranes from rabbit distal colon

Active Na+ absorption by tight epithelia such as frog skin and distal colon share common features like feedback inhibition of cellular [Na+] on Na+ influx through amiloride-sensitive Na+ channels. It is postulated that the negative feedback of increasing cell [Na+] is mediated via a rise in cell [Ca2+]. In this model, cell [Na+] is coupled to cell [Ca2+] via a Na+/Ca2+ exchange system in the basolateral membrane. In the present study, the Ca2+ transporting systems in rabbit distal colon basolateral membranes were characterized. ATP-dependent Ca2+ uptake could be demonstrated in membrane vesicles from surface cells with the following kinetic parameters: Km = 0.09 microM Ca2+ and Vm = 3.8 nmol Ca2+ mg-1 protein min-1. The ATP-dependent Ca2+ transport was not responsive to ruthenium red and oxalate, suggesting a plasmalemmal origin. The addition of 75 mM Na+ to the uptake medium, 10 min after addition of ATP, did not release Ca2+ from the vesicles in significant amounts. In the absence of ATP, outwardly directed Na+ gradients were incapable of stimulating Ca2+ uptake. This study demonstrates that rabbit distal colon epithelium lacks a well-defined Na+/Ca2+ exchange system, and (Ca2+, Mg2+)-ATPase appears to be the sole Ca2+ extrusion system. Alternatives for the coupling of cell [Na+] to cell [Ca2+] are discussed.