Renin regulation in cultured proximal tubular cells

Recent studies have documented the presence of a complete renin- angiotensin system in the proximal tubule of the kidney; however, little is known about the regulation of renin in this proximal tubular system. Therefore, we performed the present studies to learn whether the behavior of the renin system in cultured proximal tubule is similar to that of the juxtaglomerular renin system. Basal renin secretion from rabbit proximal tubular cells in primary culture was low and not affected by isoproterenol (10^-5 mol/L), diltiazem (10^-5 mol/L), or a zero-calcium bath (0 nmol/L). Only the calcium ionophore A23187 (10^-4 mol/L) significantly reduced renin secretion in these cells (from 2.44±0.37 to 1.14±.08 ng angiotensin I/mg protein per hour, P<.05). When the proximal tubular cells were lysed so the effects of the test agents on intracellular renin content could be assessed, isoproterenol caused a significant twofold (107%) increase (from 2.02±0.56 to 4.18±0.81 ng angiotensin I/mg protein per hour, P<.05), whereas diltiazem, A23187, and zero- and high-calcium baths did not produce a significant change. The effects of these agents on renin mRNA were examined in rabbit and rat proximal tubular cells in primary culture with the use of an S₁ nuclease protection assay. Densitometry analysis of renin mRNA and either GAPDH mRNA (rat) or α-actin (rabbit) showed no significant alterations in renin mRNA abundance. In summary, these results confirm the presence of renin mRNA in cultured proximal tubular cells and suggest that a low-level, constitutive secretion of renin occurs in this system that is decreased by A23187. Moreover, the results also suggest that proximal tubular renin is regulated, albeit differently from the juxtaglomerular renin system. Finally, short-term increments in proximal tubular renin occur without a change in renin mRNA.