The calcium entry blocker verapamil increases red cell flux in the vasa recta of the exposed renal papilla

Voltage-gated divalent currents in descending vasa recta pericytes

Multiple voltage-gated Ca(2+) channel (Ca(V)) subtypes have been reported to participate in control of the juxtamedullary glomerular arterioles of the kidney. Using the patch-clamp technique, we examined whole cell Ca(V) currents of pericytes that contract descending vasa recta (DVR). The dihydropyridine Ca(V) agonist FPL64176 (FPL) stimulated inward Ca(2+) and Ba(2+) currents that activated with threshold depolarizations to -40 mV and maximized between -20 and -10 mV. These currents were blocked by nifedipine (1 μM) and Ni(2+) (100 and 1,000 μM), exhibited slow inactivation, and conducted Ba(2+) > Ca(2+) at a ratio of 2.3:1, consistent with "long-lasting" L-type Ca(V). In FPL, with 1 mM Ca(2+) as charge carrier, Boltzmann fits yielded half-maximal activation potential (V(1/2)) and slope factors of -57.9 mV and 11.0 for inactivation and -33.3 mV and 4.4 for activation. In the absence of FPL stimulation, higher concentrations of divalent charge carriers were needed to measure basal currents. In 10 mM Ba(2+), pericyte Ca(V) currents activated with threshold depolarizations to -30 mV, were blocked by nifedipine, exhibited voltage-dependent block by diltiazem (10 μM), and conducted Ba(2+) > Ca(2+) at a ratio of ∼2:1. In Ca(2+), Boltzmann fits to the data yielded V(1/2) and slope factors of -39.6 mV and 10.0 for inactivation and 2.8 mV and 7.7 for activation. In Ba(2+), V(1/2) and slope factors were -29.2 mV and 9.2 for inactivation and -5.6 mV and 6.1 for activation. Neither calciseptine (10 nM), mibefradil (1 μM), nor ω-agatoxin IVA (20 and 100 nM) blocked basal Ba(2+) currents. Calciseptine (10 nM) and mibefradil (1 μM) also failed to reverse ANG II-induced DVR vasoconstriction, although raising mibefradil concentration to 10 μM was partially effective. We conclude that DVR pericytes predominantly express voltage-gated divalent currents that are carried by L-type channels.

Influence of captopril on red cell velocity in the vasa recta of the renal medulla

Previous studies from our laboratory have indicated an important role for angiotensin II (AII) in the regulation of renal medullary haemodynamics during normal physiological conditions. In order to investigate further the influence of endogenous AII on the juxtamedullary vascular resistance in anaesthetized rats the velocity of fluorescently-labeled red cells (vRBC) was measured with a cross-correlation technique in the vasa recta before and after infusion of the angiotensin I- converting enzyme inhibitor captopril or vehicle. Irrespective of treatment, vRBC was higher in the descending vasa recta (DVR) than in the ascending vasa recta (AVR). In time control animals VRBC in DVR and AVR and mean arterial blood pressure (MAP) remained stable over the 45 min study period. In animals receiving captopril (3 mg.h-1.kg-1 bw) vRBC increased almost proportionally in DVR and AVR; by 26% in DVR (from 1.02 +/- 0.12 to 1.28 +/- 0.10 mm.s-1, p < 0.05) and by 19% in AVR (from 0.46 +/- 0.05 to 0.55 +/- 0.07 mm.s-1, p < 0.05). MAP decreased by 9% (from 107 +/- 3 to 97 +/- 2 mm Hg, p < 0.05). These results give further support to the suggestion of an involvement of AII in the regulation of juxtamedullary vascular resistance during normal physiological conditions.

Tubuglomerular feedback control in long-looped nephrons. Topical minireview

The tubuloglomerular feedback mechanism is highly activated in juxtamedullary nephrons and considered to play a major role in intrarenal regulation of glomerular filtration rate. The vasculature of juxtamedullary nephrons is highly vasoreactive with a high ability for vasodilation. This vasoreactivity is a prerequisite for an important influence of the tubuloglomerular feedback mechanism on the medullary blood flow and its regulation.

Influence of verapamil on regional renal blood flow: a study using multichannel laser-Doppler flowmetry

In anaesthetized male Sprague-Dawley rats the influence of the calcium entry blocker verapamil (2.4 mg h-1 kg-1, i.v.) on renal superficial cortical (CO) and outer medullary (OM) blood flow was investigated with the aim of elucidating further the intrarenal heterogeneity in vasoreactivity. The blood flow of the two regions was monitored simultaneously with a laser-Doppler flowmeter, using fibre probes with an outer diameter of 0.75 mm. One probe was directed towards the cortex and a second probe was inserted through the cortex and positioned in the outer medulla, measuring the flow within a hemisphere with a depth of 0.5-1.0 mm. The OM probe position was verified by dissecting the kidneys after each experiment. Insertion of the OM probe did not affect whole-kidney glomerular filtration rate, renal plasma flow or electrolyte excretion. Thirty minutes of verapamil infusion increased OM blood flow by 26%, but did not change CO blood flow (-1%). In spite of the increase in OM blood flow, the urine osmolality remained unaltered. Sodium excretion increased by 39%, while potassium excretion was unchanged. Mean arterial blood pressure decreased by 13%. In conclusion, this study has further supported the suggestion that the vasoreactivity is higher in the juxtamedullary than in the superficial cortical vasculature. The heterogeneity of the response is most probably due to the previously documented pressure drop along the interlobular arteries, which will create different haemodynamic conditions for the juxtamedullary and superficial afferent arterioles.

Effect of papillary exposure on intrarenal distribution of glomerular filtration rate and of plasma flow

Removal of the renal pelvis in order to expose the renal papilla has been shown to cause impairment of the renal concentrating ability by an unknown mechanism. To study this phenomenon, urine osmolality (Uosm), single-nephron glomerular filtration rate (SNGFR) in outer and inner cortical nephrons and the inner and outer medullary plasma flow were determined. Measurements were performed on groups of rats before (control) and 15, 45 and 90 min after exposure of the left renal papilla. Rats with an intact ureter were studied in parallel to see whether the variables varied within the 90-min period of the study. In all groups of animals with an exposed papilla, Uosm was lower than in non-exposed animals. Outer cortical SNGFR in rats with exposed papillae, regardless of time, was not different from that in control rats. Inner cortical SNGFR after 45 and 90 min of exposure did not differ from that in controls, but after 15 min of exposure it was lower than in control animals. Outer and inner medullary plasma flow did not differ between rats with exposed papillae and controls, irrespective of exposure time. In conclusion, papillary exposure results in a permanent decrease in urine osmolality. This impairment of the concentrating ability cannot be attributed to prolonged changes in renal haemodynamics.