Estimation of non-autonomic and autonomic components of iliac bed vascular resistance in renal hypertensive rabbits.

Iliac bed vascular resistance (IVR) was measured before and after pharmacological block of the autonomic effectors in unanaesthetized renal hypertensive and normotensive rabbits with previously implanted Doppler flowmeters. This permitted partitioning the resting IVR into a non-autonomic component (ie, steady-state IVR after block) and an autonomic component (ie, resting IVR minus non-autonomic IVR). When IVR was measured at the same mean arterial pressure (MAP) before and after block in each animal, the increase in estimated non-autonomic IVR accounted entirely for the rise in resting IVR in renal hypertensive rabbits. However, if IVR measurements after block were made at a lower MAP than before block, the estimated non-autonomic and autonomic components were both significantly increased in renal hypertensive rabbits. It is concluded that in the latter experiment non-autonomic IVR in renal hypertension was underestimated, whilst the autonomic component was overestimated. The rise in non-autonomic IVR in renal hypertension was partly due to structural changes in the iliac bed, since IVR remained higher in hypertensive than in normotensive rabbits after abolishing smooth muscle tone with vasodilator drugs.