Contractile Response of Aorta to α1-Adrenergic Stimulation in Ca2+-Free Medium Is Reduced in Spontaneous Hypertension

Vascular responses of aortic rings to alpha 1-adrenergic stimulation by phenylephrine (Phe) from spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were studied in Ca(2+)-containing medium and Ca(2+)-free medium plus 50 mumol/L EGTA. Although there was no difference in the sustained force development between SHR and WKY vessels in response to 100 mmol/L KCl or 10 mumol/L Phe in Ca(2+)-containing medium, the transient contractile response to 10 mumol/L Phe in Ca(2+)-free medium was substantially smaller in SHR compared to that in WKY. Subsequent addition of 2.5 mmol/L Ca2+ restored the sustained contractile response to a similar level in both SHR and WKY vessels. The transient contractile response to Phe in Ca(2+)-free medium containing EGTA, presumably due to the release of intracellular Ca2+, decreased progressively with preincubation time in Ca(2+)-free medium, indicating intracellular Ca2+ depletion. Such a temporal change of aortic response was more pronounced in SHR than in WKY. The subsequent response to Ca2+ repletion in the presence of Phe, on the other hand, increased progressively with Ca(2+)-depletion period and was higher in SHR than in WKY. The rate of relaxation after washout of Phe was slower in SHR aorta compared to WKY aorta. These results, together with our earlier findings, collectively suggest that the previous known deficiency in Ca2+ pumping mechanisms of vascular muscle microsomes leading to a reduced functional size of intracellular Ca2+ pool may account for the smaller contractile response of SHR aorta to alpha 1-adrenergic stimulation in Ca(2+)-free medium and the slower rate of relaxation.