INTERSTITIAL POTENTIAL DURING PROPAGATION IN BATHED VENTRICULAR MUSCLE

Theoretical simulations have suggested that interstitial potential (V(is)) during action potential propagation affects measurements of the transmembrane action potential in bathed ventricular muscle. To evaluate the V(is) experimentally, we obtained V(is) and intracellular action potential (V(ic)) recordings at various depths in paced guinea pig papillary muscles bathed in oxygenated Tyrode's solution. The peak-to-peak amplitude and the maximum dV/dt (dV/dt(max)) of the intrinsic downward deflection of the V(is) recordings were determined. The transmembrane action potential (TM) was obtained by subtracting each V(is) from the corresponding V(ic) recording, and measurements for the phase zero depolarization and action potential foot of the V(ic) were compared with the measurements for the TM. At penetration depths of approximately 54-mu-m, the amplitude and dV/dt(max) of the V(is) were 13 mV and -38 V/s. When the depth was increased to 200-mu-m, these parameters increased to 24 mV and -59 V/s (P < 0.005), and when the depth was further increased to 390 mu-m, the parameters decreased to 16 mV and -38 V/s. Because of the V(is) at the various depths, the V(ic) underestimated dV/dt(max) of phase zero of the TM by 20-31%, which would reduce estimates of Na+ current obtained from dV/dt. Also, the V(ic) overestimated the time constant of the 2-8 mV foot of the action potential by 48-82%, which would reduce estimates of the "effective" membrane capacitance by 33-45%. These influences of the V(is) on measurements may affect results of quantitative studies of the ventricular action potential.