Developmental changes in the electrophysiologic properties of rabbit papillary muscles

We studied the electrophysiological properties of adult (AD) and newborn (NB) rabbit papillary muscles in vitro with superfusion of normal Tyrode's solution, solutions with elevated [K+]o, and in solutions with various concentrations of tetrodotoxin. In control solutions, the NB papillary muscles had a more negative resting membrane potential (-83.6 +/- 1.2 versus -80.0 +/- 1.5 mV), a higher rate of rise of phase 0 (134 +/- 5 versus 120 +/- 5 V/S) and a higher, longer-lasting action potential plateau than the AD papillary muscles. Exposure to elevated [K+]o led to a significant post-repolarization refractoriness in AD papillary muscles that was more than that for NB papillary muscles even when NB papillary muscles were depolarized to the same resting membrane potential as the AD papillary muscles. The NB papillary muscles were comparatively resistant to tetrodotoxin in terms of percent reduction of conduction velocity and percent rise in the current threshold for excitation. The conduction velocity for AD papillary muscles in control solution (66 +/- 6 cm/s) was more than for NB papillary muscles (44 +/- 4 cm/s), which would not be expected from the data on the rate of rise of the action potential, suggesting that the cable properties of NB papillary muscles (specifically a greater surface to volume ratio of the ventricular cells) are also significantly different from the AD papillary muscles.