Changes in myocardial capillary diffusion capacity during infusion of vasoactive drugs

The present study investigated how variations in coronary vascular resistance and metabolic demand affected myocardial capillary diffusion capacity. Hearts from Wistar rats were perfused with Krebs-Henseleit-albumin buffer in a Langendorff preparation, where heart rate (HR), contractility (dP/dtmax) and myocardial oxygen consumption (MVO2) were recorded continuously. Myocardial capillary diffusion capacity was measured as the permeability surface area product (PS) for Cr-EDTA and vitamin B12 by the single injection colorimetric indicator dilution method. After base-line recordings without drugs, angiotensin II+Arginine-vasopressin was infused, which increased coronary vascular resistance by 90%, stimulated HR by 11%, decreased dP/dtmax by 21% and reduced MVO2 by 4%. PSCr-EDTA and PSB12 decreased by 24 and 27%, respectively, leaving the ratio PSCr-EDTA/PSB12 unchanged indicating unaltered capillary permeability. Moreover, the reductions in MVO2 and PS correlated significantly. During vasodilation: (1) nitroprusside-NA stimulated HR by 7% and decreased dP/dtmax by 14%; (2) adenosine reduced dP/dtmax by 37% and decreased MVO2 by 9%; and (3) isoproterenol increased HR, dP/dtmax and MVO2 by 53, 76 and 9%, respectively. However, all three vasodilators reduced PSCr-EDTA and PSB12 in parallel by 7-25% leaving PSCR-EDTA/PSB12 unchanged. Thus, maximal estimated diffusion capacities were obtained during spontaneous coronary vascular tone, most likely reflecting maximal capillary recruitment in the Krebs-Henseleit-albumin perfused heart. The derecruiting effects of the vasoconstrictors were partly overridden by metabolic factors, while the reductions of PS after vasodilation more likely were due to increased heterogeneity in coronary flow.