Dependence on external calcium for the noradrenaline contractility of the resistance vessels in spontaneously hypertensive and renal hypertensive rats, as compared with normotensive controls.

Isolated hindquarters of rats were perfused at constant flow with a plasma substitute so that pressor responses to various concentrations of noradrenaline (NA) could be measured in consective sections of the vascular bed with normal (greater than 1.5 mM) or low (0.2 mM) Ca2+ in the perfusate. The animals used were 6-week-old spontaneously hypertensive rats of the Okamoto strin (SHRy), normotensive controls of the same age NCRy); 6--7-month-old SHR (SHRA) and NCR (NCRA); and rats made hypertensive by clipping of one renal artery (RHR) plus matched normotensive controls (NCRR). Concentration-response curves to NA showed that constrictor responses to NA become more dependent on external calcium as one proceeds peripherally in the vascular system in both SHRA and NCRA, with responses in proximal vessels being least and small pre- and postcapillary vessels being most dependent in both cases. In low calcium SHRA retained their responses better than NCRA, whereas RHR retained their responses to NA poorer than did NCRR. No significant differences in responses in low calcium were observed in SHRY compared to NCRY though a trend in the same direction as in SHRA could be traced. It is concluded that there is no evidence that altered handling of calcium initiates vascular hyperreactivity in SHR, but that the handling of vascular calcium in SHR differs from RHR and both differ from NCR.