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.
Wiener
H
H
Department of Physiology, University of Nijmegen, The Netherlands.
de Jong
M D
MD
van Os
C H
CH
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D066298:In Vitro Techniques; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000187:drug effects / Q000378:metabolism*; D007474:Ion Exchange; D008297:Male; D011188:Potassium / Q000378:metabolism; D011817:Rabbits; D012964:Sodium / Q000378:metabolism / Q000502:physiology
D012964:Sodium; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D011188:Potassium; D002118:Calcium