Extracellular Ca2+ regulates the stimulation of Na+ transport in A6 renal epithelia.
We investigated the involvement of intracellular and extracellular Ca2+ in the stimulation of Na+ transport during hyposmotic treatment of A6 renal epithelia. A sudden osmotic decrease elicits a biphasic stimulation of Na+ transport, recorded as increase in amiloride-sensitive short-circuit current (Isc) from 3.4 +/- 0.4 to 24.0 +/- 1.3 microA/cm2 (n = 6). Changes in intracellular Ca2+ concentration ([Ca2+]i) were prevented by blocking basolateral Ca2+ entry with Mg2+ and emptying the intracellular Ca2+ stores before the hyposmotic challenge. This treatment did not noticeably affect the hypotonicity-induced stimulation of Isc. However, the absence of extracellular Ca2+ severely attenuated Na+ transport stimulation by the hyposmotic shock, and Isc merely increased from 2.2 +/- 0.3 to 4.8 +/- 0.7 microA/cm2. Interestingly, several agonists of the Ca2+-sensing receptor, Mg2+ (2 mM), Gd3+ (0.1 mM), neomycin (0.1 mM), and spermine (1 mM) were able to substitute for extracellular Ca2+. When added to the basolateral solution, these agents restored the stimulatory effect of the hyposmotic solutions on Isc in the absence of extracellular Ca2+ to levels that were comparable to control conditions. None of the above-mentioned agonists induced a change in [Ca2+]i. Quinacrine, an inhibitor of PLA2, overruled the effect of the agonists on Na+ transport. In conclusion, we suggest that a Ca2+-sensing receptor in A6 epithelia mediates the stimulation of Na+ transport without the interference of changes in [Ca2+]i.
Jans
Danny
D
Laboratory of Physiology, Biomedical Research Institute, Limburgs Universitair Centrum, Universitaire Campus, B-3590 Diepenbeek, Belgium.
Simaels
Jeannine
J
Larivière
Els
E
Steels
Paul
P
Van Driessche
Willy
W
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002460:Cell Line; D004553:Electric Conductivity; D005110:Extracellular Space / Q000378:metabolism; D007038:Hypotonic Solutions / Q000494:pharmacology; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D009994:Osmolar Concentration; D009997:Osmotic Pressure; D010741:Phospholipases A / Q000378:metabolism; D054467:Phospholipases A2; D044169:Receptors, Calcium-Sensing / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D012964:Sodium / Q000378:metabolism*; D019459:Urothelium / Q000166:cytology / Q000378:metabolism*; D014867:Water / Q000378:metabolism; D014982:Xenopus laevis
D007038:Hypotonic Solutions; D044169:Receptors, Calcium-Sensing; D014867:Water; D012964:Sodium; D010741:Phospholipases A; D054467:Phospholipases A2; D008274:Magnesium; D002118:Calcium