Theoretical simulations have suggested that interstitial potential (Vis) during action potential propagation affects measurements of the transmembrane action potential in bathed ventricular muscle. To evaluate the Vis experimentally, we obtained Vis and intracellular action potential (Vic) 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/dtmax) of the intrinsic downward deflection of the Vis recordings were determined. The transmembrane action potential (TM) was obtained by subtracting each Vis from the corresponding Vic recording, and measurements for the phase zero depolarization and action potential foot of the Vic were compared with the measurements for the TM. At penetration depths of approximately 54 microns, the amplitude and dV/dtmax of the Vis were 13 mV and -38 V/s. When the depth was increased to 200 microns, these parameters increased to 24 mV and -59 V/s (P less than 0.005), and when the depth was further increased to 390 microns, the parameters decreased to 16 mV and -38 V/s. Because of the Vis at the various depths, the Vic underestimated dV/dtmax of phase zero of the TM by 20–31%, which would reduce estimates of Na+ current obtained from dV/dt. Also, the Vic 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 Vis on measurements may affect results of quantitative studies of the ventricular action potential.
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