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Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7545
This study
provides new information about the relative importance of
Ca2+ mobilization and entry in the
renal vascular response to adrenoceptor activation. We measured renal
blood flow (RBF) in Sprague-Dawley rats in vivo using
electromagnetic flowmetry. We measured intracellular free
Ca2+ concentration
([Ca2+]i)
in isolated afferent arterioles utilizing ratiometric photometry of
fura-2 fluorescence. Renal arterial injection of NE produced a
transient decrease in RBF. The response was attenuated, in a dose-dependent manner, up to ~50% by nifedipine, an antagonist of
L-type Ca2+ entry channels.
Inhibition of Ca2+ mobilization by
3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8)
inhibited the renal vascular effects of NE in a dose-dependent manner,
with maximal blockade of ~80%. No additional attenuation was
observed when nifedipine and TMB-8 were administered together. In
microdissected afferent arterioles, norepinephrine (NE;
10
6 M) elicited an
immediate square-shaped increase in
[Ca2+]i,
from 110 to 240 nM. This in vitro response was blocked by nifedipine
(106 M) and TMB-8
(105 M) to a degree similar
to that of the in vivo experiments. A nominally calcium-free solution
blocked 80-90% of the
[Ca2+]i
response to NE. The increased
[Ca2+]i
elicited by depolarization with medium containing 50 mM KCl was totally
blocked by nifedipine. In contrast, TMB-8 had no effect. Our results
indicate that both Ca2+ entry and
mobilization play important roles in the renal vascular Ca2+ and contractile response to
adrenoceptor activation. The entry and mobilization mechanisms
activated by NE may interact. That a calcium-free solution caused a
larger inhibition of the NE effects on afferent arterioles than
nifedipine suggests more than one Ca2+ entry pathway.
norepinephrine; adrenoceptor; renal circulation; afferent arteriole; 3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester; nifedipine; vascular smooth muscle
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