[The control of renin secretion]

Renin release and autoregulation of blood flow in a new model of non-filtering non-transporting kidney

1. A recently developed model of a non-filtering, non-transporting dog kidney, obtained by an in situ filling of tubules with low-viscosity oil, was applied for studies of renin release and autoregulation of renal blood flow (RBF). 2. Renal blood flow was partially autoregulated after oil blockade of tubules, as indicated by a mean autoregulation index (Semple-de Wardener (1959) of 0-5. This was comparable to autoregulation of the stop-flow kidney (index 0-6) and contrasted with abolition of autoregulation after hypertonic mannitol loading at stop-flow conditions (index 1-1). 3. The aortic construction at a suprarenal level, which decreased renal perfusion pressure of the oil-blocked kidney 35 +/- (S.E. of mean) 6 mmHg, produced an increase in arterial plasma renin activity of 1-8 +/- 0-1 ng. ml.-1 (P less than 0-02). Renin secretion rate decreased 33 to 70 ng.min-1 in three dogs in which renal perfusion pressure was reduced to 60--66 mmHg, but increased 110 +/- 41 ng.min-1 when pressure reductions were kept within the renal blood flow autoregulation range (n=8, P less than 0-025). 4. These results suggest that signals from the tubular receptor (macula densa) are not necessary for stimulation of renin release or autoregulation of renal blood flow.

Evidence for hemodynamic autoregulation of renin release

To examine the relationship between renal arteriolar dilatation and renin release, arterial perfusion pressure in anesthetized dogs was lowered in steps within and below the range of autoregulation of renal blood flow. Renin release, determined by both bioassay and radioimmunoassay, averaged 2.7 ± 0.9 ( SE ) µg/min at control pressure and increased to 20.0 ± 4.1 ( SE ) µg/min at the lowest autoregulating pressure, which averaged 66.4 ± 2.9 ( SD ) mm Hg. However, renin release then remained constant during further lowering of arterial perfusion pressure despite reductions in renal blood flow. This response was not significantly changed when sodium excretion was increased by intravenous infusion of mannitol. In another series of experiments, renin release was raised by reducing arterial perfusion pressure below the range of autoregulation, and control sodium excretion was reestablished by mannitol or saline infusion; renin release remained high. Therefore, renin release appears to be related to autoregulated dilatation of the renal arterioles, and it becomes maximal when the arterioles are completely dilated at the lowest pressure of autoregulation of renal blood flow. This mechanism is different from release mechanisms dependent on sodium delivery to the distal nephron.

Quantification of intermediate steps in the renin-angiotensin-vasoconstrictor feedback loop in the dog

The major intermediate steps in the renin-angiotensin-vasoconstrictor feedback loop have been experimentally determined. The quantitative relationships between renal perfusion pressure, renin secretion, arterial renin activity, and systemic arterial blood pressure were determined in dogs in which the cardiovascular control loops of the central nervous system were eliminated by spinal cord destruction and decapitation. Step-decreases in renal perfusion pressure to a single kidney were introduced and maintained constant to open the feedback loop of arterial pressure. Renin activity was measured by radioimmunoassay of angiotensin I. Each decrease of 15 mm Hg in renal perfusion pressure between pressures of 100 and 50 mm Hg elevated the net secretion of renin nearly 20 ng min -1 g -1 kidney an d the arterial renin activity nearly 7.0 ng ml -1 hour -1 . Renin secretion and arterial renin activity decreased at perfusion pressures below 50 mm Hg. A bioassay procedure for estimating the rate of angiotensin II formation at various increments of arterial renin activity showed that an increase in renin activity of 10 ng ml -1 hour -1 resulted in an increase in the net production of angiotensin II of 5.0 ng kg -1 min -1 . The results of these experiments are useful in predicting alterations in the system that follow a decrease in renal artery pressure, and they clarify interactions of the renin-angiotensin system with other homeostatic pressure-regulating systems.