Effects of protamine on myocyte contractile function and β-adrenergic responsiveness

Abstract

The use of protamine sulfate in patients has been associated with severe circulatory collapse and myocardial failure. However, the exact mechanisms responsible for these reactions to protamine remain unclear. Accordingly, we examined the effect of protamine on isolated myocyte contractile function. Indexes of isolated myocyte contractile function, percent shortening, and velocity of shortening were examined using videomicroscopy. Porcine cardiocytes (n = 75) were studied at baseline and in the presence of 80 μg/mL protamine. In addition, myocyte function was examined sequentially, first during treatment with 8 IU/mL heparin and then after the addition of a protamine dose sufficient to completely bind the heparin. The binding of heparin and protamine resulted in the formation of a heparin-ptotamine complex. The protamine concentration of 80 μg/mL is approximately equal to the serum concentration of protamine obtained in patients when administered in a dose of 5 mg/kg. In the presence of 80 μg/mL protamine, both percent shortening and velocity of shortening fell by more than 32% from baseline values ( p < 0.05). The presence of either heparin alone or the heparinprotamine complex resulted in no change in baseline myocyte contractile measurements. Furthermore, to examine the effect of protamine on myocyte β-adrenergic responsiveness a second series of experiments were performed. Myocyte contractile function was measured when 25 nmol/L isoproterenol was added to each of the protocols above. The presence of 80 μg/mL protamine resulted in a significant blunting of myocyte μ-adrenergic responsiveness. The presence of either heparin alone or the heparin-protamine complex resulted in no change in myocyte β-adrenergic responsiveness. In summary, the presence of unbound protamine but not the heparinprotamine complex resulted in depressed baseline myocyte contractile function and blunted myocyte β-adrenergic responsiveness. This study demonstrated that unbound protamine can directly depress myocyte contractile performance. Thus, one potential mechanism for the alteration in hemodynamics and left ventricular function that can occur after the administration of protamine is the direct depressant effect of unbound protamine on myocyte contractile function.