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1 Section of Hypertension and Vascular Research, University of Texas Medical Branch, Galveston, Texas 77555-1065 and 2 Julius L. Chambers Biomedical Biotechnology Research Institute, North Carolina Central University, Durham, North Carolina 27707
Renal interstitial fluid Ca2+
concentration ([Ca2+]isf) was
measured in anesthetized Wistar rats by using in situ microdialysis. During perfusion of 20 cm of the proximal small intestine with Ca2+-free buffer, renal
[Ca2+]isf was 1.63 ± 0.19 mmol/l in the cortex (n = 6) and 1.93 ± 0.12 mmol/l in the medulla (n = 5, P = 0.223). When
Ca2+ in the intestinal lumen was increased to 3 mmol/l, no
change was seen in total or ionized serum Ca2+
(SCa), urinary Ca2+ excretion
(UCa), or Ca2+ in a microdialysate of the
kidney cortex. Increasing intestinal Ca2+ further, to 6 mmol/l, was without effect on SCa but significantly increased UCa by 38% and microdialysate Ca2+
by 36% (1.25 ± 0.0.09 vs. 1.70 ± 0.14 mmol/l, n = 4, P < 0.05). Intravenous infusion of 28 ng
· kg
1 · min1
of parathyroid hormone for 1 h during perfusion of the intestinal lumen
with 1 mmol/ Ca2+caused a 7-10% rise in
SCa, a 40% fall in UCa, and a 32% increase in
microdialysate Ca2+ (1.32 ± 0.13 vs. 1.74 ± 0.13 mmol/l, n = 6, P < 0.05). Interlobar arteries with a
mean diameter of 120 µm were studied by using a wire myograph to
determine whether changes in extracellular Ca2+ affect
muscle tone. When precontracted with 5 µmol/l serotonin, the arteries
relaxed in response to cumulative addition of Ca2+
(1-5 mmol/l) with an ED50 value for Ca2+
of 3.30 ± 0.08 mmol/l, n = 3. These data demonstrate that
[Ca2+]isf changes dynamically
during manipulation of whole-animal Ca2+ homeostasis and
that intrarenal arteries relax in response to extracellular
Ca2+ varied over the range measured in vivo.
kidney; calcium; microdialysis; relaxation
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