Effects of mechanical stretch on the membrane potential of guinea pig ventricular muscles.

Abstract

We studied the effect of stretch on the membrane potentials and ultrastructure of isolated ventricular papillary muscles of guinea pigs. The muscles were stimulated at 0.5 Hz and stretched stepwise from slack length (90% of Lmax) to 100% (mild stretch), 110-120% (moderate stretch), and 130-140% of Lmax (severe stretch), under microscopic control. In control Tyrode solution (K + = 5.4 mM, Ca2 + =1.8 mM, Mg2 + = 0.5 mM), the mild to moderate stretch significantly depolarized the resting potential (RP) by about 6 mV as compared to that in slack length, whereas the severe stretch hyperpolarized the membrane by about 5 mV. The latter finding was new and was focused on in later experiments. Both the hyperpolarization and depolarization became more marked when [K+]0 was decreased to 1.35-2.7 mM, and became less with elevated [K+]o to 10.8--21.6 mM, thereby suggesting the participation of altered K+ conductance (gK) with these changes in the RP. Perfusion with low [Ca2 +]o (0.45 mM) enhanced the depolarization but eliminated the hyperpolarization; high [Ca2 +]o (7.2 mM) inhibited the depolarization without effect on the hyperpolarization. D-600 (1 /IM), caffeine (10 mM), and ryanodine (1 µM), all of which may produce decreases in [Ca2 +]i, abolished the hyperpolarization with inconsistent effects on the depolarization. Moderate to severe stretches decreased the maximum rate of rise of action potential (Vmax)' by shifting the Vmax'M relationship toward hyperpolarizing direction. The shift could be reversed partially after increasing [Mg2 +]o to 8.0 mM. Electron microscopic examination revealed that the sarcoplasmic reticulum (SR) remained intact with mild to moderate stretches with significant lengthening of sarcomere length, while with a severe stretch, the SR showed a structural disarrangement with a non-uniform lengthening of sarcomere length. Our observations suggest that stretch-induced hyperpolarization is probably mediated by the increase in gK, presumably secondary to the increase in

Effects of elastase on contractility and morphology of elastic tissue in isolated guinea pig papillary muscles

“…The muscle length corresponding to 50 mg increase of RT in the control condition (Fig. 5, left) was equivalent to the stretch of up to about 120% of Lma x (a stretch by 20%), as reported previously [17].…”

“…Thus, the use of this preparation is suitable for examining the relationship between sarcomere length and contractile tension. The relationship between resting tension (RT) and muscle length is found to be essentially linear within a certain range of muscle length in the guinea pig ventricular papillary muscle [17]. RT of the preparation is maintained, for the most part, by the parallel elastic component of the contractile protein.…”

“…After a 30-min exposure to ELA (3 x 10-4g/ml), we observed that this layer was partially disrupted and lost its continuity, as shown in Fig. 6D. closely related to the muscle length and, therefore, to the sarcomere length [17]. In order to obtain the Lma×, RT was adjusted to produce the maximum TT by finely manipulating the position of the strain gauge.…”

Mechanisms of Stretch-Induced Changes in [Ca 2+ ] i in Rat Atrial Myocytes

“…Many reports indicate that stretch causes a depolarization of the RP of cardiac myocyte. 3,7,72 In the present study the RP of the myocytes was not significantly altered by moderate stretch, neither in experiments nor in the model. The reason for this was that only a very small SA current (depolarizing current) is needed to reproduce the normal physiological time-dependent changes in rat atrial function.…”

Influence of stretch on excitation threshold of single frog ventricular cells

“…Theoretical (Kim, 1992;Ruknudin et al 1993;Van Wagoner, 1993) or chloride channel (Hagiwara et al 1992;Sorota, 1992;Tseng, 1992) recently reported. Another scenario is that non-selective mechanosensitive channels having a more positive reversal potential may exist, but their effect is countered by another stretch-sensitive potassium channel, which would tend to hyperpolarize the cell membrane, as has been described in guinea-pig papillary muscle (Nakagawa, Arita, Shimada & Shirabe, 1988). These and other possibilities are discussed again later in this section.…”