Renal interstitial hydrostatic pressure and ANF in exaggerated natriuresis of the SHR

The Okamoto spontaneously hypertensive rat (SHR) has exaggerated natriuretic and diuretic responses to acute volume expansion in comparison with the Wistar-Kyoto (WKY) rat. The objective of these experiments was to determine the role of renal interstitial hydrostatic pressure (RIHP) and atrial natriuretic factor (ANF) in these natriuretic and diuretic responses of the SHR and the WKY rat. Two groups each of SHRs and WKY rats were used in this study. In one group of each, both kidneys were acutely decapsulated, and in the remaining two groups the renal capsules were left intact (control). In both control SHR (n = 6) and WKY (n = 7) groups, volume expansion was associated with a significant increase in RIHP (from 5.0 ± 0.5 to 7.1 ± 0.6 mmHg in SHR group and from 6.0 ± 0.5 to 9.4 ± 0.3 mmHg in WKY rat group) and a high level of plasma ANF (P(ANF)). The increase in RIHP was significantly lower in the control SHR group compared with that of control WKY rat group (2.1 ± 0.4 vs. 3.5 ± 0.3 mmHg; P < 0.05), whereas P(ANF) was significantly higher in the SHR group (848 ± 151 pg/ml) compared with that of the WKY group (503 ± 37 pg/ml) at the end of the volume-expansion period. When the increase in RIHP that occurs during volume expansion in the SHR is blunted by acute bilateral renal decapsulation, the natriuretic and diuretic responses are significantly attenuated despite levels of P(ANF) that are similar to those measured in the control SHR group. We conclude that elevations in RIPH and P(ANF) are important contributors to the natriuretic and diuretic responses of the SHR and WKY rats to acute saline volume expansion.