Endothelial inhibition of myofilament calcium response in intact cardiac myocytes

Recent studies suggest that factors released by endothelial cells can modify contraction of isolated cardiac preparations. We compared the effects of 1) coronary effluent collected from Langendorff-perfused rat hearts and 2) cultured vascular endothelial cell superfusate on isolated fura 2-loaded rat ventricular cardiac myocytes. Coronary and cultured cell effluent produced similar effects. Isotonic contraction amplitude was reduced by 31.6 +/- 2.6 and 70.2 +/- 9.1%, respectively; myocyte diastolic length increased by 0.8 +/- 0.2 and 1.5 +/- 0.4 microns, and time to 50% relaxation fell by 6.2 +/- 1.8 and 10.1 +/- 2.0% (all P < 0.05; n = 29 and 15 myocytes, respectively). A small fall in the amplitude of the intracellular Ca2+ transient was observed (8.5 +/- 1.5 and 10.9 +/- 3.5%, respectively; both P < 0.01), insufficient to account for the reduction in twitch amplitude. In intact myocytes tetanized in the presence of thapsigargin, the steady-state myofilament response to Ca2+ was reduced by coronary and cultured cell effluent. These results suggest that both coronary endothelial cells in situ and cultured endothelial cells tonically release a factor(s) that reduces myofilament Ca2+ response.