The cellular cardiac effects of protamine, the cationic polypeptide 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㎍/ml) were compared to those of heparin 910, 30 units/ml), and to heparin (10 units/ml) neutralized with equivalent (100 ㎍/ml)
or
excess (200㎍/ml) protamine. The effects of protamine were also examined using muscle rapid cooling contractures (RCCs to assess intracellular Cs2 stores). Protamine (110-300㎍/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 o.1 ㎛ isoproterenol, 30-300㎍/ml protamine caused dose-dependent depression of late 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 ㎍/ml protamine. Effects of heparin with 100 ㎍/ml excess protamine were similar to those of 100㎍/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 Ca2+, mediated by an alteration in memebrane ionic conductances.
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