Myocardial inotropic responses to aggregating platelets and modulation by the endocardium

Ventricular mural thrombi complicate many cardiac diseases. The endocardial endothelium can modulate the mechanical performance of subjacent myocardium and mediate responses to certain physiopharmacologic agents. We studied the effects of aggregating platelets on the contractile performance of isolated cardiac muscle. The role of the endocardium was investigated by selectively damaging it by very brief (1 second) exposure to 1% Triton X-100 in some muscle preparations before experiments. Cat papillary muscles (n = 54) were attached to an electromagnetic length-tension transducer in organ baths containing Krebs-Ringer solution (1.25 mM Ca2+, 35 degrees C), and stimulated electrically at 0.2 Hz. Homologous washed platelets (final concentration 3 x 10(11)/l) aggregated spontaneously on addition to baths. Mechanical performance increased significantly more in muscles with damaged endocardium than in intact muscles (p less than 0.05); total peak isometric twitch tension increased by 31.8 +/- 7.8% (with damaged endocardium) and 11.8 +/- 2.6% (with intact endocardium), and peak isotonic twitch shortening increased by 36.7 +/- 7.8% (with damaged endocardium) and 9.6 +/- 2.0% (with intact endocardium). Increases in maximum velocity of unloaded shortening were similar in both muscle groups. Time to half isometric twitch tension decline decreased in intact muscles (3.6 +/- 1.0%) but increased in Triton-treated muscles (2.5 +/- 1.3%, p = 0.003 for difference between groups). The inotropic response to platelets in muscles with intact endocardium was unaltered by pretreatment of muscles with indomethacin (10 microM) or by stimulation of platelet aggregation with thrombin (0.1 unit/ml).(ABSTRACT TRUNCATED AT 250 WORDS)