1. Calcium reactivity of arterial smooth muscle and the effect of diltiazem (calcium antagonist) were investigated in helical strips of mesenteric arteries from 6-week-old male spontaneously hypertensive rats (SHR) and age-and sex-matched normotensive Wistar-Kyoto (WKY) rats. Blood pressure was 130 ± 5 mmHg for SHR and 110 ± 5 mmHg for WKY rats (means ± sd).

2. At high Ca2+ concentration (3 × 10−4 to 3 × 10−3 mol/l) the calcium-induced tension of mesenteric arterial strips per cross-sectional area was significantly greater in SHR than in WKY rats in noradrenaline (10−5 mol/l)-containing solution or in high K+ (121 mmol/l) solution (P < 0.001). At low Ca2+ concentration (10−5 mol of noradrenaline/l and 3 × 10−5 mol/l in high K+), the tension was smaller in SHR strips than in WKY rat strips (P < 0.05).

3. The Ca2+ sensitivity of SHR strips was significantly lower than that of WKY strips in noradrenaline-containing solution or in high K+ solution.

4. Diltiazem, a calcium antagonist, shifted the dose-response curve of Ca2+-induced tension to the right equally in SHR and WKY rat strips. The inhibition of the maximum tension by the drug was greater in SHR strips than in WKY rat strips in noradrenaline-containing solution, but there was no difference in high K+ solution.

5. The results suggest that the contractile abnormality might be a possible cause of hypertension in SHR.

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