Cardiotoxic Depressant Effects of Protamine

Park, Wyun Kon; Bang, Sou Ouk; Hong, Yong Woo; Shin, Ho Sun

Abstract

The cellular cardiac effects of protamine, the cationic polypeptide employed to reverse heparin anticoagulation, were examined in vitro to define its mechanisms of action. Isometric contractile force and action potential (AP) characteristics after rest (RS) and at frequencies up to 3 Hz were recorded in guinea pig ventricular papillary muscle. The actions of protamine (10-300 ug/ ml) were compared to those of heparin (10, 30 units/ml), and to heparin (10 units/ml) neutralized with equivalent (100 ug/ml) or excess (200 ug/ml) protamine. The effects of protamine were also examined using muscle rapid cooling contractures (RCCs to assess intracellular Ca(z+) stores). Protamine (100-300 ug/ml) depressed contractions by 35-65% at 3 Hz, whereas contractions were enhanced 150-500% at lower rates (RS-0.5 Hz), with a concommitant rise in resting force. Protamine caused a resting depolarization from -84 to -72 mV and depressed AP amplitude. In contrast, heparin minimally altered contractile or AP characteristics. In 26 mM K(+)-solution with 0.1 uM isoproterenol, 30-300 ug/ml protamine caused dose-dependent depression of late peaking force development and slow AP prolongation. After 15 minutes rest, when RCCs were not normally elicited, rest RCCs became prominent in 100-300 ug/ml protamine. Effects of heparin with 100 ug/ml excess protamine were similar to those of 100 ug/ml protamine alone. In conclusion the loss of normal force-frequency relation, partial depolarization, rise in resting tension, and appearance of rested state RCCs suggest that unbound protamine can lead to excess intracellular Ca(2+), mediated by an alteration in memebrane ionic conductances.