Effects of isoprenaline on functional capillary density in the subendocardial and subepicardial layer of the rat myocardium.

The functional capillary density in subepicardial and subendocardial layers of rat heart was measured during rest and during isoprenaline-induced (5.0 microgram X kg-1 X min-1, i.v. over 3 minutes) cardiac stimulation. For determination of the number of perfused capillaries, a fluorescent dye (thioflavine S) was infused into the left atrium; 1, 3, 5 and 10 sec, respectively, after starting dye application, hearts were excised and rapidly cooled down to -50 degrees C. In histological sections capillaries which had been perfused during the dye infusion could be identified and counted. An increase in the number of stained vessels was found in both layers of the myocardium when the time of dye exposure was prolonged. Under these conditions the rise was much smaller in isoprenaline-treated animals, this effect being most marked in the subendocardial layer (3560 +/- 199 cap./mm2, control group; 2190 +/- 30 cap./mm2, isoprenaline-treated group; dye exposure 10 sec). Isoprenaline - at the dose used - induced an increase in total blood flow (3.7 +/- 0.6 ml X min-1 X g-1, control group; 6.8 +/- 0.7 ml X min-1 X g-1, isoprenaline-treated group), however, with a relatively less pronounced increase in the subendocardial blood flow (subendocardial/subepicardial flows: 1.08 +/- 0.13, control group; 0.66 +/- 0.01, isoprenaline-treated group). These results favour the view that isoprenaline-induced relative reduction in the subendocardial blood flow is due to disturbance of perfusion pressure and extravascular compression rather than to exhaustion of the myocardial capillary reserve.