Measurement of blood flow to the adrenal capsule, cortex and medulla in dogs after hemorrhage by fluorescent microspheres

Abstract

Changes in adrenal medullary and total cortical blood flow after hemorrhage have been described using radioactive microspheres. To assess changes in adrenal capsular and in intracortical adrenal blood flow, a method was used based on microscopic detection of non-radioactive microspheres. Injection of microspheres labelled with fluorescent dyes permitted multiple determinations of blood flow. Pentobarbital anesthetized dogs ( n = 6) were prepared acutely with left ventricular and aortic catheters for injection and collection of microspheres, respectively. Adrenal denervation was done unilaterally by cutting the thoracic splanchnic nerve. Injections of 16-μm spheres were made prior to and immediately after 18 ml/kg hemorrhage done over 6 min. Dogs were killed with KCI and adrenals were removed, fixed and sectioned at 80 μm. Using fluorescence microscopy, microspheres were counted in the adrenal capsule, zona glomerulosa, inner cortex (zona facsiculata and reticularis), and the medulla. The majority (95%) of microspheres in the adrenal cortex were trapped in the zona glomerulosa, precluding an independent estimate of blood flow to the inner cortex. Thus, total cortical blood flow was determined by summing the number of 16-μm microspheres in the zona glomerulosa and inner cortex. Prior to hemorrhage, blood flow was greater in the capsule (5.4 ± 1.6 ml/min/g) compared to the cortex (1.8 ± 0.9 ml/min/g) and the medulla (2.9 ± 1.8 ml/min/g). Splanchnicotomy did not change blood flow in the resting state. Following hemorrhage, in innervated glands, medullary blood flow increased to 8.6 ± 3.1 ml/min/g, whereas blood flow to other zones was unchanged. Since the mean arterial pressure observed after hemorrhage varied among dogs, the relationship between blood flow in each adrenal zone and the mean arterial pressure in individual dogs was assessed by regression analysis. After hemorrhage, blood flow and vascular conductance in each adrenal zone were inversely proportional to mean arterial pressure. In the medulla, splanchnicotomy abolished sensitivity of blood flow to mean arterial pressure, whereas in capsule and cortex splanchnicotomy did not affect sensitivity of blood flow to mean arterial pressure. These findings support earlier work showing that the distribution of adrenal blood flow changes after hemorrhage and is influenced by splanchnic innervation. In addition, these findings suggest that blood flow to the adrenal capsule and cortex may be maintained during hypotension independently of thoracic splanchnic innervation.