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.