Systolic pressure and the myogenic response of the renal afferent arteriole.

The transmission of elevated blood pressure to the glomerulus and pressure-induced glomerular injury play central roles in the pathogenesis of kidney disease and its progression to end-stage renal failure. The renal afferent arteriole sets the pre-glomerular resistance and pressure-induced or 'myogenic' afferent arteriolar vasoconstriction is a primary mechanism protecting the glomerulus from the damaging effects of hypertension. The systolic pressure, being the highest level of pressure attained and most frequent pressure oscillation impacting on the renal vasculature, potentially represents the most damaging component of the blood pressure. Indeed, recent studies indicate that elevations in systolic blood pressure are more closely linked to kidney disease than are elevations in diastolic pressure. However, the current view, derived from dynamic studies of autoregulation, is that the renal vasculature responds passively to pressure signals presented at rates exceeding the myogenic operating frequency (0.2-0.3 Hz in the rat). Thus existing concepts do not explain the mechanisms that normally protect the kidney from elevations in the systolic pressure which are presented at the heart rate (6 Hz in the rat). A recent study from our laboratory addressed this issue. Using a modelling approach and direct measurements of myogenic responses, we found that the afferent arteriole is able to sense and appropriately adjust tone in response to changes in systolic pressure, presented at the heart rate. Key kinetic attributes allowing this vessel to respond in this manner appear to be a very short delay in activation, an unusually rapid rate of vasoconstriction and a longer delay in vasodilation. The present review summarizes this work and presents recent findings addressing the determinants of the myogenic vasoconstriction in the afferent arteriole.