Myocytes Isolated From Rejecting Transplanted Rat Hearts Exhibit Reduced Basal Shortening Which is Reversible by Aminoguanidine

Transplanted hearts exhibit depressed contractile function during periods of acute rejection. Myocytes from rejecting hearts also express inducible nitric oxide synthase (iNOS). We hypothesized that an intrinsic defect, due to the increased nitric oxide production by myocytes, is responsible for much of the observed contractile dysfunction. To test our hypothesis, we recorded shortening of myocytes isolated from rejecting (allograft) and non-rejecting (isograft) transplanted rat hearts under control conditions and following exposure to aminoguanidine (an inhibitor of iNOS), or methylene blue (an inhibitor of nitric oxide stimulation of guanylate cyclase). Four days after transplantation, basal shortening was reduced in allograft myocytes compared to isograft myocytes (allografts: 7.0±0.8μm; isografts: 10.7±0.9μm;P<0.05). Allograft myocytes also had higher cGMP levels than isograft myocytes (allografts: 0.58±0.16 pmol/mg protein; isografts: 0.13±0.08 pmol/mg protein;P<0.05). Aminoguanidine (1 m ) had no effect on shortening or cGMP levels in isograft myocytes, whereas aminoguanidine significantly reduced cGMP levels and greatly enhanced shortening of allograft myocytes, such that shortening was now similar in allograft and isograft myocytes. Methylene blue (100μ ) also caused a more than three-fold greater increase in shortening of allograft myocytes (+80±15%) than isograft myocytes (+23±6%;P<0.05 from allografts). These results suggest that myocytes isolated from rejecting hearts have a reversible intrinsic contractile depression which is mediated by overstimulation of the nitric oxide/cGMP pathway within the myocytes. This intrinsic contractile dysfunction may be a major factor responsible for the reversible cardiac depression associated with acute rejection of transplanted hearts.