We have measured the calcium sensitivity in response to noradrenaline stimulation and potassium depolarization of isolated segments of 100 to 200 micrometer mesenteric resistance vessels from spontaneously hypertensive (SHR) and control Wistar-Kyoto rats. The rats were either young (4 wk) or adult (4 months), that is of ages before or after the SHRs had developed elevated blood pressure. Experiments were performed under conditions in which the effect of noradrenaline uptake by, and the potassium-induced noradrenaline release from, the nerve terminals in the vessel walls was eliminated. 2. The response of the SHR and WKY vessels to noradrenaline under conditions where only the extracellular calcium appeared to have been removed was similar. When subsequently stimulated maximally by noradrenaline, the calcium-sensitivity of the SHR vessels (Ca-ED50 approximately or equal to 0.1 mM) was greater than that of the WKY vessels (Ca-ED50 approximately or equal to 0.2 mM). When depolarized by potassium, all vessels were less sensitive to calcium and there was little difference in the calcium sensitivity of SHR and WKY vessels in either age group (Ca-ED50 approximately or equal to 0.8 mM). 3. The results suggest that whereas the potassium (potential)-dependent calcium permeability of the SHR smooth muscle cell membrane is normal, the noradrenaline-induced calcium permeability is abnormally high. The presence of this abnormality in the vessels from the young SHRs suggests that it may be a factor involved in the aetiology of hypertension in the SHR.
