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

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Abstract
To investigate the vasoreactivity of the vessels of the juxtamedullary vasculature, measurements were made of the red blood cell flux (Qrbe) in the vasa recta of the exposed renal papilla of anaesthetized rats by means of fluorescently labelled erythrocytes. The flux was measured before and after intravenous administration of the calcium entry blocker verapamil, which was infused in two different doses. A control group receiving vehicle was studied in parallel. The lower dose of verapamil (0.6 mg h‐lkg‐1body wt) increasedQrbeby 30 ± 6% (n =9,P <0.05, mean ± I SEM) and reduced the systemic blood pressure by 6% (P <0.05). The osmolality of urine samples from the papillary tip remained statistically unchanged (1199 ± 60vs. 1186 ± 102 mosm kg‐1H2O). In the animals receiving the higher dose of verapamil (2.4 mg h‐1kg‐1body wt), Qrbcincreased by 39 ± 7% (n =10,P0.05) and the blood pressure was reduced by 9% (P <0.05), while urine osmolality was unchanged (1104 ± 110vs. 1264 ± 123 mosm kg‐1H2O). In the time control group there was no statistically significant change in Qrbe(+ 6 ± 8%,n= 10), blood pressure or urine osmolality (1006 ± 61vs. 1091 ± 70 mosm kg‐1H2O).The verapamil‐induced increase inQrbcis probably mainly mediated by a relaxation of the afferent arteriole of the juxtamedullary nephrons. The results confirm our previous indication of a high vasoreactivity in the vessels of the juxtamedullary vasculature. This vasoreactivity is a prerequisite for the large regulatory span of the renal medullary blood flow and consequently for the medullary function.