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In open-chest dogs (n = 5) the effects of nifedipine (25 micrograms/kg infused over a period of 15 min) on the collateral blood flow after left anterior descending coronary artery (LAD) ligation were separated from those on heterogeneous blood supply, i.e. flow of collateral origin plus flow in non-ischemic myocardium projecting into the ischemic area. This separation was possible using a technique based on microspheres, allowing analysis of perfusion via the left anterior descending coronary artery (LAD). Secondary effects on the ischemic heart induced by the hypotensive effect of nifedipine per se were minimized by counterbalancing the blood pressure change with an intra-aortic balloon. Nifedipine increased non-ischemic myocardial blood flow in the subendocardium from 1.11 +/- 0.10 to 2.71 +/- 0.17 (p less than 0.001) and in the subepicardium from 1.12 +/- 0.08 to 3.95 +/- 0.49 (p less than 0.001) (ml/min/g, mean +/- SE). Collateral blood flow in the centre of the ischemic area was not affected by nifedipine. In the subendocardium, it was 0.12 +/- 0.05 before and 0.09 +/- 0.09 after nifedipine, and in the subepicardium, the corresponding values were 0.31 +/- 0.10 and 0.29 +/- 0.04, respectively. At the lateral ischemic border, the nifedipine-induced increase in myocardial blood flow was only directed to the admixed normal tissue. When the blood flow was corrected for this overlapping non-ischemic tissue, no significant effect of nifedipine was measurable in the subendocardial blood flow, which was 0.12 +/- 0.03 before and 0.18 +/- 0.04 ml/min/g after drug administration.(ABSTRACT TRUNCATED AT 250 WORDS)