1. The sensitivity of isolated tail artery strips from adult spontaneously hypertensive (SH) and normotensive rats to endogenously released and exogenously applied noradrenaline was compared. Release or displacement of endogenous noradrenaline was obtained with electrical stimulation, tyramine, elevated potassium and potassium-free solution. Isometric contractile responses were measured before and after acute denervation with 6-hydroxydopamine or before and after treatment with phentolamine.
2. The sensitivity to exogenous noradrenaline of innervated arterial strips was similar for SH and normotensive rats. Acute denervation produced a significant shift to the left in the concentration-response curve to noradrenaline only in SH rat arterial strips.
3. Contractile responses to electrical stimulation and tyramine were similar in both groups before denervation. Contractile responses to potassium-free solution were greater in SH than in normotensive arterial strips. After denervation the SH and normotensive vessels responded similarly to these interventions. Contractile responses to elevated potassium were similar in both groups before and after treatment with phentolamine.
4. The results suggest that the sensitivity of vascular smooth muscle cells to noradrenaline is greater in denervated arterial strips from SH rats than in those from normotensive rats. Under most experimental conditions, the junctional concentration of noradrenaline released or displaced from the adrenergic nerve endings is less in arterial strips from SH than in those from normotensive rats. Apparently, the adrenergic neuroeffector nerve terminals in hypertensive blood vessels can modulate the junctional concentration of noradrenaline so that the contractile response to this agent is similar to that in normotensive blood vessels.