Role of calcium in insulin-stimulated nacl transport in the
medullary thick ascending limb.
Ito, Osamu, Yoshiaki Kondo, Nobuyuki Takahashi, Ken Omata, and Keishi
Abe.
Department of Clinical Biology and Hormonal Regulation, Pediatrics
and Second Department of Internal Medicine, Tohoku University School
of Medicine, Seiryo-machi 1-1, Sendai 980, Japan
APStracts 2:0038F, 1995.
It has been reported that insulin stimulates directly NaCl transport
in the rabbit medullary thick ascending limb (MTAL) (Ito, O. et al.,
Am. J. Physiol. 267: F265-F270, 1994). In the present investigation,
we evaluated the role of Ca2+ in insulin-stimulated NaCl transport in
rabbit MTAL by in vitro microperfusion methods. In control
experiments, insulin increases transepithelial voltage (Vte) and net
lumen-to-bath Cl- flux (JCl). The effects of insulin on Vte and JCl
in a Ca2+-free solution containing EGTA did not differ from those in
a Ca2+-containing control solution. Direct measurements of cytosolic
free Ca2+ ([Ca2+]i) with fura-2 fluoresence showed that insulin
caused no detectable change in [Ca2+]i in MTAL cells. Chelation of
intracellular Ca2+ with BAPTA/AM inhibited the actions of insulin in
Vte and JCl without affecting basal values. We examined further
whether calmodulin is also involved in insulin-stimulated NaCl
transport in MTAL using two dissimilar inhibitors of calmodulin,
trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphtholene
sulphonamide (W-7). TFP and W-7 inhibited the action of insulin in a
dose-dependent manner with maximal inhibition of both agents being
> 90%. The half-maximal inhibition by TFP and W-7 was approximately
50 and 100 [mu]M, respectively. These results suggest that insulin
-stimulated NaCl transport in MTAL is mediated through the
intracellular Ca2+-calmodulin system, but not through Ca2+ influx.
Received 29 December 1994; accepted in final form 14 March 1995.
APS Manuscript Number F462-4.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 28 March 1995.