Effect of acute stimulation of renin secretion on renal renin content in vivo

Acute stimulation of renal renin secretion has been reported to increase, not change, or decrease intra-renal renin (IRR) content. However, these effects and the potential mechanisms for acute changes in IRR content have not been studied directly. In this study, the effect of acute stimulation of renin secretion on IRR content was studied directly using a new in vivo blood perfused rabbit kidney preparation. Following removal of the right kidney for determination of IRR content (N = 7) the left kidney was cannulated and perfused for an average of 55 minutes (baseline) at mean arterial blood pressure. Renin secretion by the left kidney was subsequently stimulated by reducing renal perfusion pressure to 60 mm Hg and administering enalapril, 1 mg/kg intravenously. After 38 to 140 minutes (mean 90 min) of stimulation, the left kidney was also removed and IRR content assessed. Renal blood flow and renin secretory rate (RSR) were determined frequently at baseline and following stimulation of renin secretion. The total amount of renin secreted in response to acute stimulation was calculated by integrating the RSR response over time. RSR from the left kidney increased by 515% during acute stimulation. IRR content in the left kidney also increased and averaged 16% greater than the right kidney from the same animal. In order to account for all of the renin secreted as well as the increase in IRR content following acute stimulation, it was calculated that the renin synthesis rate would have been required to increase over 26-fold. Alternatively, the increase in IRR content observed could have resulted from activation of a previously synthesized pool of inactive renin. Thus, acute stimulation of renin release in this in vivo model is accompanied by an increase in IRR content which can be explained by a large increase in renin synthesis and/or intra-renal activation of inactive renin.