We investigated the alterations in basal tone of aortic strips by changing the Ca concentration, basal 45Ca uptake and 3H-nitrendipine binding of the single cells of aortic smooth muscles in the spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats. While the basal tone of the aortic strips in WKY rats was not affected by alteration of Ca concentration, that in SHR was decreased by the removal of Ca from the bath solution and was recovered by the restoration of Ca to normal levels. This contraction increased in a Ca concentration-dependent manner and reached a maximum at 2 mM Ca. The basal tone of aorta in SHR was suppressed by verapamil (10-6M). The basal tone of aorta in SHR increased about 50% in the strips of endothelial rubbing, compared with that of intact endothelium. Basal 45Ca uptake in the aortic single smooth muscle cells of SHR was greater than that of WKY (p<0.01), Specific bindings of [3H]nitrendipine in the aortic single smooth muscles of SHR and WKY were saturable. The dissociation constant (Kd) was 0.71±0.15 and 1.18±0.08nM SHR, respectively, and the difference in Kd between two strains was statistically significant (p<0.03). The maximal binding capacity (Bmax) was 34.6±3.2 and 47.4±4.3 fmol/106 SHR respectively, and the difference of (Bmax) between two strains was statistically significant (p<0.05). from the above results, it is suggested that the increase of Ca influx via potential-operated Ca channels and the increase of the number of dihydropyridine-sensitive Ca channels contribute to high basal tone of the aortic strips in SHR.
