Resetting of tubuloglomerular feedback in acute volume expansion in rats

Acute saline expansion increases nephron filtration and distal flow rate but maintains tubuloglomerular feedback responsiveness: role of adenosine A(1) receptors

Temporal adaptation of tubuloglomerular feedback (TGF) permits readjustment of the relationship of nephron filtration rate [single nephron glomerular filtration rate (SNGFR)] and early distal tubular flow rate (V(ED)) while maintaining TGF responsiveness. We used closed-loop assessment of TGF in hydropenia and after acute saline volume expansion (SE; 10% body wt over 1 h) to determine whether 1) temporal adaptation of TGF occurs, 2) adenosine A(1) receptors (A(1)R) mediate TGF responsiveness, and 3) inhibition of TGF affects SNGFR, V(ED), or urinary excretion under these conditions. SNGFR was evaluated in Fromter-Wistar rats by micropuncture in 1) early distal tubules (ambient flow at macula densa), 2) recollected from early distal tubules while 12 nl/min isotonic fluid was added to late proximal tubule (increased flow to macula densa), and 3) from proximal tubules of same nephrons (zero flow to macula densa). SE increased both ambient SNGFR and V(ED) compared with hydropenia, whereas TGF responsiveness (proximal-distal difference in SNGFR, distal SNGFR response to adding fluid to proximal tubule) was maintained, demonstrating TGF adaptation. A(1)R blockade completely inhibited TGF responsiveness during SE and made V(ED) more susceptible to perturbation in proximal tubular flow, but did not alter ambient SNGFR or V(ED). Greater urinary excretion of fluid and Na(+) with A(1)R blockade may reflect additional effects on the distal nephron in hydropenia and SE. In conclusion, A(1)R-independent mechanisms adjust SNGFR and V(ED) to higher values after SE, which facilitates fluid and Na(+) excretion. Concurrently, TGF adapts and stabilizes early distal delivery at the new setpoint in an A(1)R-dependent mechanism.

Renal blood flow control by tubuloglomerular feedback (TGF) in normal and spontaneously hypertensive rats--a role for dopamine and adenosine

Following the elementary laws of hemodynamics and the functional characteristics of the renal myogenic and macula densa-mediated (TGF) vascular resistance control mechanisms, TGF-mediated changes of renal vascular resistance are amplified by cooperative changes of the myogenic mechanism. Myogenically induced changes, on the other hand, would be antagonized by TGF. Resetting of renal vascular flow resistance by alterations to the TGF mechanisms might thus be more effective than alterations to the myogenic mechanism. Dopamine and adenosine, two autacoids occurring normally in the tubular fluid, may play a key role in operating such a resetting mechanism. Dopamine and adenosine were found in proximal tubular fluid at concentrations of 10(-8) and 0.5 10(-6) M respectively. Dopamine inhibits the tubuloglomerular feedback mechanism, this inhibition is antagonized concentration-dependently by adenosine. These effects most likely occur via D1 and A1 receptors and hence by regulation of the adenyl cyclase activity in the macula densa cells. The balance between adenosine and dopamine in tubular fluid appears to be under the control of extrarenal parameters. In normal rats, high dietary salt intake, by influencing the secretion of an unknown adrenal hormone, and inhibition of Na-K-ATPase might be of importance. In spontaneously hypertensive rats unknown genetic parameters may also play a role.

Blood pressure and tubuloglomerular feedback mechanism in chronically salt-loaded spontaneously hypertensive rats

Experiments were performed to qualitatively characterize the effects of tubuloglomerular feedback (TGF) inhibition by chronic salt loading on salt sensitivity of blood pressure in spontaneously hypertensive rats (SHR). After two weeks of salt loading, systolic blood pressure (SBP) was significantly exacerbated and plasma volume (PV) was expanded in salt-loaded SHR compared with those in control SHR (SBP: 182 +/- 1 vs. 159 +/- 2 mm Hg; PV: 4.38 +/- 0.06 vs. 4.04 +/- 0.03 ml/100 g body wt, respectively). Plasma volume of WKY was also but only transiently expanded by salt loading, whereas plasma volume expansion in SHR had persisted over the entire dietary treatment period. TGF activity was assessed as the maximal reduction of single nephron GFR (SNGFR) on increasing loop of Henle perfusion rate from 0 to 40 nl/min using previously collected tubular fluid from salt-loaded rats (TFs) or control rats (TFc). Maximal TGF response in salt-loaded SHR with TFs was 14.9 +/- 2.9% and 57.8 +/- 2.6% with TFc. In control SHR the responses were 16.9 +/- 2.5% with TFs and 52.7 +/- 2.9% with TFc. In salt-loaded WKY the response with TFs were 3.1 +/- 1.6% and 37.4 +/- 2.8% with TFc. And in control WKY, the response with TFs were 8.2 +/- 1.9% and 40.8 +/- 2.8% with TFc, respectively. These results indicate the TGF resetting in chronically salt-loaded SHR and WKY is caused by the activation of humoral TGF inhibitory factor. The suppression of TGF in SHR was, however, far more variable and, on average, less than in WKY.(ABSTRACT TRUNCATED AT 250 WORDS)