Intravenously injected metaraminol induced a larger blood pressure increase in hypertensive spontaneously rats (SHR) than in normotensive controls (NR) when the pressure was raised from the same starting level. Cerebral blood flow (CBF) response in NR was either perfect autoregulation, partial autoregulation or “break-through.” When present, the autoregulatory response was very rapid, i.e. the flow returned to the initial value within 10-15 sec. All SHR showed an initial prompt vasoconstrictor response which was followed after 30-40 sec by a gradual flow increase. The blood pressure elevation was highest in SHR when hypertension was induced by compression of the aorta, which supports the hypothesis that the enhanced response is, at least in part, a consequence of an increased vessel wall to lumen ratio. The characteristic CBF pattern observed in SHR after a metaraminol-induced rise in blood pressure was not seen when the blood pressure was increased by aortic compression, which suggests an effect of the drug separate from its pressor effect.
During maximum vasodilatation the cerebrovascular resistance (CVR) was considerably higher in SHR than in NR. Assuming an equivalent vessel density in the 2 groups, our results suggest that structural changes in resistance vessels in SHR encroach on the lumen.