Eukaliuric diuresis and natriuresis in response to the KATP channel blocker U37883A: micropuncture studies on the tubular site of action

Abstract

1. Systemic application of U37883A, a blocker of ATP sensitive potassium (KATP) channels, elicits diuresis and natriuresis without significantly altering urinary potassium excretion. 2. To elucidate tubular sites of action upstream to the distal nephron, micropuncture experiments were performed in nephrons with superficial glomeruli of anaesthetized Munich-Wistar-Frömter rats during systemic application of U37883A (1, 5 or 15 mg kg-1 i.v.). 3. The observed eukaliuric diuresis and natriuresis in response to U37883A at 15 mg kg-1 was accompanied by an increase in early distal tubular flow rate (VED) from 10 - 18 nl min(-1) reflecting a reduction in fractional reabsorption of fluid up to this site (FR-fluid) of 13%. The latter proposed an effect on water-permeable segments such as the proximal tubule which could fully account for the observed reduction in fractional reabsorption of Na+ up to the early distal tubule (FR-Na+) of 8% and the increase in early distal tubular Na+ concentration ([Na+]ED) from 35 - 51 mM whereas [K+]ED was left unaltered. 4. In comparison, furosemide (3 mg kg-1 i.v.), which acts in the water-impermeable thick ascending limb, elicited diuresis, natriuresis and kaliuresis which were associated with a fall in FR-Na+ of 10% with no change in FR-fluid, and a rise in [Na+]ED from 42 - 117 mM and [K+]ED from 1.2 - 5.7 mM with no change in VED. 5. Direct late proximal tubular fluid collections confirmed a significant inhibition of fluid reabsorption in proximal convoluted tubule in response to systemic application of U37883A. 6. These findings suggest that the diuretic and natriuretic effect upstream to the distal tubule in response to systemic application of U37883A involves actions on water-permeable segments such as the proximal convoluted tubule.

Figure 1

Effect of systemic application of U37883A or furosemide on (i) early distal tubular flow rate and sodium and potassium ion concentration (V_ED, [Na⁺]_ED, [K⁺]_ED) as well as (ii) urinary fluid, sodium ion and potassium ion excretion (V_urine, Na⁺_urine, K⁺_urine). (a) Schematic illustration of the nephron (with glomerulus, proximal tubule, loop of Henle and distal tubule) and of micropuncture manoeuvres. (b and c) Depicted is the effect of the experimental manoeuvre (EXP: i.v. administration of Vehicle, U37883A, or furosemide on the respective parameters as compared to basal measurement (Basal). Mean±s.e.mean n is given as number of nephrons/number of rats; ^*P<0.05 vs basal. Notice that the diuresis and natriuresis in response to U37883A at 15 mg kg⁻¹ was associated with an increase in [Na⁺]_ED and V_ED. In comparison, the diuresis, natriuresis and kaliuresis in response to furosemide, which acts in the water-impermeable TALH, was associated with a substantial rise in [Na⁺]_ED and [K⁺]_ED without altering V_ED. U37883A at 5 mg kg⁻¹ elicited a similar response up to early distal tubule as 15 mg kg⁻¹ (as indicated by similar rises in V_ED and [Na⁺]_ED), which, however, was associated with an attenuated diuresis and natriuresis and the presence of kaliuresis.

Figure 2

Effect of systemic application of U37883A on single nephron filtration rate (SNGFR) and proximal convoluted tubule fluid reabsorption as determined by free-flow collection from late proximal tubular site. (a) Schematic illustration of micropuncture manoeuvres. (b) Depicted are the changes in SNGFR (Δ SNGFR), and absolute (Δ AR-fluid) and fractional (Δ FR-fluid) reabsorption of fluid in the proximal convoluted tubule in response to systemic application of U37883A or Vehicle as compared to basal measurements. Mean±s.e.mean. ^*P<0.05 vs basal; Δ FR-fluid: #P<0.05 vs vehicle. Notice that systemic application of U37883A caused a significant inhibition of fluid reabsorption in the proximal convoluted tubule. Whereas during fluid collection from the early distal tubular site, systemic application of U37883A left the SNGFR unaltered (Table 1), an increase in SNGFR was observed during fluid collection from the late proximal tubular site.

Figure 3

Effect of systemic application of U37883A on proximal convoluted tubule fluid reabsorption as determined by stopped-flow perfusion. (a) Schematic illustration of micropuncture manoeuvres. (b) Depicted are the changes in absolute (Δ AR-fluid) and fractional (Δ FR-fluid) reabsorption of fluid in the proximal convoluted tubule in response to systemic application of U37883A as compared to Vehicle treatment. Mean±s.e.mean. #P<0.05 vs vehicle. Notice that systemic application of U37883A significantly reduced fluid reabsorption in proximal convoluted tubule during stopped-flow perfusion.

Figure 4

Effect of intratubular application of U37883A on proximal convoluted tubule fluid reabsorption during stopped-flow perfusion. (a) Schematic illustration of micropuncture manoeuvres. (b) Depicted are the changes in absolute (Δ AR-fluid) and fractional (Δ FR-fluid) reabsorption of fluid in the proximal convoluted tubule in response to intratubular application of U37883A or vehicle as compared to basal measurements. Mean±s.e.mean. ^*P<0.05 vs basal. Notice that intratubular application of U37883A significantly inhibited fluid reabsorption in the proximal convoluted tubule.

Figure 5

Effect of intratubular application of U37883A on loop of Henle reabsorption during stopped-flow perfusion. (a) Schematic illustration of micropuncture manoeuvres. (b) Depicted is the effect of the experimental manoeuvre (EXP: intratubular application of Vehicle or U37883A) on early distal tubular flow rate (V_ED) and sodium and potassium ion concentration ([Na⁺]_ED, [K⁺]_ED) as compared to basal measurement (Basal). Experiments were performed in Munich-Wistar-Frömter (MWF) and Sprague-Dawley (SD) rats. In contrast to SD rats, MWF rats have nephrons with superficial glomeruli which allow to puncture the early distal tubule closer to the macula densa. Mean±s.e.mean. ^*P<0.05 vs basal. Notice that intratubular application of U37883A did not significantly alter V_ED [Na⁺]_ED, or [K⁺]_ED in MWF or SD rats.