On the Mechanisms of Post-Rest Adaptation in the Isolated Electrically Driven Left Atria of Rats

Abstract

We studied the role of the resting period (1, 2, 4, 8, 16 min; n = 6-7), external Ca2+ (0.2, 0.4, 0.6 g/l; n + 5-6), stimulation frequency (1, 2, 3 Hz; n = 6), 4-aminopyridine (4-AP, 2 mM; n = 5); theophylline (1 mM; n = 6), ouabain (5 microM; n = 6), and verapamil (1 microM; n = 6) on post-rest adaptation in the isolated left atria of rats driven electrically by a 2x threshold intensity for 2 ms. Resting periods resulted in three-phasic adaptive changes in contractility during the post-rest stimulation before normalization: P1, hypercontractile phase, an initial twitch potentiation; P2, post-rest hypocontractile decay reached after 8 to 12 single twitches; and P3, a late reactive hypercontractile phase marked less than that of P1 and gradually declining to the pre-resting level. P1 and P2 were augmented along with increasing the resting period from 1 min to 16 min, whereas t1 (time between P1 and P2) shortened and P2 and t2 (time between P2 and P3) were not affected. P1 and P3 to become more apparent after shifting the stimulation frequency from 1 Hz to 3 Hz, accompanied by a shortening of t1 and t2 (p < 0.05) and an insignificant reversal of P2. An increase in Ca2+ concentration by 2- or 3-fold at 2 Hz reduced P1 was and antagonized P2, while leaving other parameters almost unaffected. The reduction of P1 by Ca2+ became more prominent at 3 Hz. Exposure to 4-AP depressed P1 and P3 at 1 Hz, which was reversed by increasing the stimulation frequency--P3 tended to diminish, whereas t1 and t2 were shortened. Theophylline reduced P1 antagonized P2, and shortened t1 and t2 significantly, and a combination of theophylline and 4-AP augmented the effects. Ouabain increased P1 and P2 in a frequency-dependent manner; prolonged t2 at 1 Hz, but shortened t2 at higher frequencies. Verapamil inhibition of Ca2+ channels augmented P1 and t1 and reduced P2 and P3, and the effects on all three parameters were augmented by combined 4-AP/verapamil. We concluded that the post-rest adaptive changes in contractility are a consequence of phasic changes in sarcoplasmic Ca2+ concentration and that such changes reflect an imbalance between the release from and uptake into the sarcoplasmic reticulum of Ca2+ and transsarcolemmal Ca2+ loss.

The role of PLC-IP3 cascade on 4-aminopyridine (4-AP) contracture in electrically-driven rat atrial and diaphragmatic strips: new evidence by neomycin and heparin

“…Initiation of muscle contraction is achieved by mutual participation of several cellular components including elevation of cytoplasmic free Ca +2 concentration mediated by depolarization of the cell membrane and spatial activation of dihydropteridine receptors (DHPRs), calcium-induced Ca +2 release through ryanodine-sensitive Ca +2 receptors (RyRs), IP 3 /Ca +2 release receptors (IP 3 -R) and the activation of membrane enzyme PLC generating IP 3 and DAG, respectively. Experimental results repeatedly suggested that 4-AP inter-acted with depolarization process (4), dihydropteridinesensitive Ca +2 channels (4,9) and sarcoplasmic Ca2+/ ATPase (11). It was rather unexpected to explore that 4-AP induced contracture in the spontaneously beating frog ventricular strips in the absence of Ca 2+ (4), a finding indicating that the source of Ca 2+ should be intracellular in origin, namely sarcoplasmic Ca +2 stores and/or Ca +2 loosely bound to phospholipids at the inner surface of the cell membrane.…”

“…The mechanisms underlying the interaction of 4-AP with several excitable cell-lines appeared to be rather complex and have not been fully explored. Experimental results steadily suggested that the mechanisms subserving its effects at the cellular and molecular level may involve inhibition of K+-channels (1-4), promotion of transmembrane Ca 2 +-influx (3,4), release of Ca 2+ from SR (6,7), inhibition of Ca 2+ -ATPase (11) or the interaction of 4-AP with intracellular Ca 2+ -depots (3)(4)(5)(6)(7)9). Indeed, induction of contracture by 4-AP in Ca 2+ -free media strongly suggested that it interacted with SR and that the source of Ca 2+ responsible for such a contracture should be of sarcoplasmic Ca 2+ stores (7,9).…”

“…Experimental results steadily suggested that the mechanisms subserving its effects at the cellular and molecular level may involve inhibition of K+-channels (1-4), promotion of transmembrane Ca 2 +-influx (3,4), release of Ca 2+ from SR (6,7), inhibition of Ca 2+ -ATPase (11) or the interaction of 4-AP with intracellular Ca 2+ -depots (3)(4)(5)(6)(7)9). Indeed, induction of contracture by 4-AP in Ca 2+ -free media strongly suggested that it interacted with SR and that the source of Ca 2+ responsible for such a contracture should be of sarcoplasmic Ca 2+ stores (7,9). The level of intracellular Ca +2 in multiple types of mammalian cells controls an enormous number of functions including cell division, glandular secretion, and contractility of muscle cells.…”

“…It has been shown that 4-AP can induce contracture in Ca 2+ -free medium either by promoting Ca 2+ release from SR or by inhibiting reverse Ca 2+ -pumping with Ca 2+ /ATPase into the SR (6,11). Furthermore, experimental results currently suggest that post-rest potentiation (Figure 3) mediated by the powerful triggering of Ca 2+ release from the SR (3)(4)(5)(6)(7)9). In the present work, the first contraction induced by 4-AP alone was found to be reduced with respect to pre-drug contractions.…”

“…Indeed, the latter view was supported by the fact that 45 Ca 2+ -binding to phosphatidylserine monolayer in vitro was shown to be inhibited by 4-AP in a dose-dependent manner (5). Furthermore, interruption of electrical stimulation for 30 sec in cardiac muscle causes the first post-rest contraction to be more powerful than the amplitude of pre-rest contraction, a phenomenon known as post-rest potentiation which is thought to be due to accumulation of Ca 2+ in SR during resting period which causes more Ca 2+ to be released through Ryn/Ca 2+ channels and thus augments first post-rest contraction (9,10). Inhibition of Ca 2+ -ATPase in SR by 4-AP may also play a role in such a potentiation and contracture development by this agent (11).…”