Smooth muscle responses of the rat septal artery are not influenced by surrounding passive cardiac tissue.

We investigated the possible contribution of surrounding passive cardiac tissue to the smooth muscle responses of coronary arteries. The vasoactive properties of the intramyocardial septal artery (outer diameter 350-400 microns) of the rat heart were investigated when it was freed from the surrounding cardiac tissue (dissected artery) and when it remained in the left ventricle and was thus embedded in passive cardiac tissue (in situ). The changes in external diameter relative to the maximal diameter (isoproterenol) resulting from the application of 125 mM KCl and 1,000 microU/ml vasopressin were measured at 37 degrees C, a transmural pressure of 100 cm H2O, and zero flow. In the dissected septal arteries (n = 5) the maximum diameter was 402 +/- 16 microns, while during exposure to KCl and vasopressin the diameter was reduced to 66.1 +/- 4.6% and 74.2 +/- 3.0%, respectively. For the in situ arteries (n = 6), the maximal diameter was 386 +/- 28 microns, a value not statistically different from the dissected vessels and the diameters reduced to 63.2 +/- 5.5% and 65.6 +/- 7.2% due to KCl and vasopressin, respectively. The constrictions of dissected arteries and in situ arteries were statistically not different. The results show that the maximally-dilated diameter and the constriction responses of intramyocardial conduit arteries of the rat heart are not affected by the surrounding passive cardiac tissue.