SPONTANEOUS AND RENAL HYPERTENSIVE RATS - RESPONSIVENESS OF ISOLATED RIGHT AND LEFT ATRIA TO NORADRENALINE, ISOPRENALINE AND METHOXAMINE

The adrenergic responsiveness of right and left atria isolated from spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR) was studied. Right atria isolated from SHR showed subsensitivity to the chronotropic effect of noradrenaline (21.3-fold at the EC(50) level, P<0.05) and isoprenaline (12.0-fold, P<0.05). However, atria isolated from RHR did not exhibit any significant alteration (P>0.05) in sensitivity to the chronotropic effect of noradrenaline or isoprenaline. Chronotropic responsiveness to theophyline was not altered in right atria isolated from SHR or RHR. alpha(1)-Mediated inotropic responses to noradrenaline and methoxamine were blunted in left atria isolated from SHR. Left atria isolated from RHR showed supersensitivity to the inotropic effect of noradrenaline (5.4-ford at the EC(50) level, P<0.05) and subsensitivity to the inotropic effect of methoxamine (6.0-fold, P<0.05). It is concluded that the present results could, at least partially, explain the reduced cardiac output observed during established hypertension in SHR and the increased cardiac output observed in the initial phase of renal hypertension in RHR.