Secretion control for active and inactive renin: effect of ouabain on release from rabbit kidney cortex slices

Release of active and inactive renin by rabbit kidney cortex slices was investigated. Inactive renin was estimated as the increase in renin activity after acidification (pH 2.8) of slice supernatant solutions. For kidney slices incubated in complete Krebs bicarbonate buffer, the Na-K-ATPase inhibitor ouabain (100 microM) reduced the secretion of both active (-19.2%) and inactive (-78.9%) forms of renin. In low Na buffers ([Na+] = 23 mM) active renin release was increased and inactive renin was suppressed. Both of these changes were abolished by addition of ouabain (100 microM). The reduction in inactive renin secretion produced by ouabain in complete Krebs buffer did not occur in low [Na+] buffers. In zero Ca2+ buffers containing EGTA (5 mM), secretion of both active and inactive renin was increased but these changes were abolished by addition of ouabain (100 microM). Incubating kidney slices in low Na+, zero Ca2+ media revealed differences between the secretion control mechanisms for the two forms of renin. The separate stimulatory effects of low Na+ and low Ca2+ were not additive for the release of active renin and inclusion of ouabain resulted in similar secretion rates to those under control conditions. For inactive renin secretion, in the absence of Ca2+ release mechanisms still respond to reduction in Na+ with decreased secretion. Conversely, in low Na+ buffers, removal of Ca2+ still promotes inactive renin secretion. These changes were abolished by the addition of ouabain (100 microM). Slices did not change in weight during incubation in media which did not contain ouabain. Addition of this inhibitor to control buffers and low Na buffers did result in an increase in weight. This correlated with the presence of Ca2+ in the buffer and did not appear to be related to [Na+]. These studies again show that the mechanisms regulating the secretion of active and inactive renin are not identical and support the hypothesis that Na+ have differing roles to play in the regulation of these two forms of renin.