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Articles in PresS, published online ahead of print September 12, 2002
Am J Physiol Heart Circ Physiol, 10.1152/ajpheart.00247.2002
Submitted on March 20, 2002
Accepted on September 9, 2002
1 Department of Physiology and Biophysics, University of Mississippi Medical Center, Jackson, MS, USA
2 Department of Veterans Affairs Medical Center, Harvard Medical School, West Roxbury, MA, USA
* To whom correspondence should be addressed. E-mail: raouf_khalil{at}hms.harvard.edu.
Reduction of uterine perfusion pressure during late pregnancy has been suggested to trigger increases in renal vascular resistance and arterial pressure which lead to hypertension of pregnancy; however, the cellular signaling mechanisms involved are unclear. The purpose of this study was to test the hypothesis that the increased renal vascular resistance and arterial pressure during reduction of uterine perfusion pressure in late pregnancy reflect increases in [Ca2+]i and contraction of renal arterial smooth muscle. Single smooth muscle cells were freshly isolated from renal interlobular arteries of late pregnant Sprague-Dawley rats and a rat model of hypertension produced by chronic reduction in uterine perfusion pressure (RUPP) in late pregnancy. The isolated cells were loaded with fura-2 and both cell length and [Ca2+]i were measured. In cells of normal pregnant rats incubated in Hank's solution (1 mM Ca2+), resting [Ca2+]i was 63±5 nM and cell length was 65±2µ m. In RUPP rats, resting [Ca2+]i (111±7 nM) was significantly greater and cell length (46±2 mm) was shorter than pregnant rats. In pregnant rats, angiotensin II (Ang II, 10-7 M) caused a transient increase in [Ca2+]i to 414±13 nM followed by a maintained increase to 149±8 nM, and 21±1% cell contraction. In RUPP rats, the Ang II-induced [Ca2+]i transient (431±18 nM) was not significantly different from pregnant rats, but the maintained [Ca2+]i was significantly elevated to 225±9 nM and cell contraction was increased to 48±2%. Membrane depolarization by 51 mM KCl, and the Ca2+ channel agonist Bay K8644 (10-6 M), which stimulate Ca2+ entry from the extracellular space, caused maintained increases in [Ca2+]i and cell contraction that were significantly enhanced in RUPP rats compared to control pregnant rats. In Ca2+-free (2 mM EGTA) Hank's, the Ang II- and caffeine (10 mM)-induced [Ca2+]i transient and cell contraction were not significantly different between normal pregnant and RUPP rats suggesting no difference in Ca2+ release from the intracellular stores. The enhanced basal, maintained Ang II-, KCl- and Bay K8644-induced [Ca2+]i and cell contraction in RUPP rats compared to normal pregnant rats suggest enhanced Ca2+ entry mechanisms of smooth muscle contraction in resistance renal arteries and may, in part, explain the increased renal vascular resistance associated with hypertension of pregnancy.
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