Shuji Ishida, MD; Naohiko Takahashi, MD; Tetsunori Saikawa, MD; Tetsu Iwao, MD; Takao Fujino, MD; Mikiko Nakagawa, MD; Hidetoshi Yonemochi, MD; Morio Ito, MD

Abstract
Previous evidence has shown that the action potential duration of rabbit ventricular muscle cells shortens after a rest period (postrest shortening). However, there has not been much research on postrest shortening in the intact heart. We recorded transmembrane action potentials (TAPs) of isolated papillary muscle from rabbit ventricle with glass microelectrodes and monophasic action potentials (MAPs) of the rabbit left ventricular endocardium with contact electrodes. In the in vitro experiments, repetitive regular stimuli (S1) at a cycle length of 1 sec were followed by a single extrastimulus (S2) at coupling intervals (S1S2) ranging between 0.5 sec and 8 sec. The increase in the S1S2 interval resulted in a progressive shortening of the duration of TAP elicited by the S2, which was abolished by the simultaneous application of 1 mmol/L 4-aminopyridine and 2ƒ mol/L ryanodine. In the in vivo experiments, regular right ventricular pacing (S1) at a cycle length of 0.35 sec was followed by a single extrastimulus (S2) with coupling intervals (S1S2) ranging between 0.25 sec and 3 sec. The increase in the S1S2 interval also resulted in a progressive shortening of the duration of MAP elicited by the S2. This is the first report to demonstrate postrest shortening in the intact heart, which probably occurs because of a mechanism analogous to that observed in the isolated ventricular muscle.
(Jpn Circ J 1998; 62: 443-448)

Key Words: Monophasic action potentials; Transient outward current; 4-Aminopyridine; Ryanodine; Sarcoplasmic reticulum

Mailing address: Naohiko Takahashi, MD, Department of Internal Medicine I, School of Medicine, Oita Medical University, 1-1 Idaigaoka, Hasama, Oita 879-55, Japan