Effects of extracellular calcium on potassium and noradrenaline induced contractions in the aorta of spontaneously hypertensive rats--increased sensitivity to nifedipine.

Isolated preparations of the thoracic aorta from spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR) were contracted by noradrenaline (NA) 1.8 X 10(-5)M, and potassium (K+) 127 mM, after 30 min. pretreatment in a Ca++-free medium. In both SHR and NWR aortae, the contractions were markedly reduced; no significant differences were found between the two types of vessels in Ca++-free medium. On addition of Ca++, the contractions were restored to a significantly greater extent in the NWR than in the SHR aortae. In the presence of nifedipine 7.2 X 10(-9)M, the response to Ca++ was significantly more reduced in the SHR than in the NWR preparations. Relaxation of NA and K+ contracted preparations was induced by wash-out of the contractile agents, by addition of nifedipine 2.9 X 10(-8)M, and by introduction of a Ca++-free medium. After wash-out of NA, relaxation was slower in SHR than in NWR vessels. Relaxation induced by nifedipine and Ca++-free medium was more complete in SHR than in NWR preparations. After wash-out of K+, relaxation was more rapid in NWR than in SHR aortae. Nifedipine and Ca++-free medium induced relaxation was more complete in SHR than in NWR preparations. The results suggest that in the SHR aortae contraction induced by NA and K+ is more dependent on extracellular Ca++ than is the response in the NWR preparations; the SHR vessels are also more sensitive to the relaxing effects of nifedipine.