CHARACTERISTICS OF TETANIC CONTRACTIONS IN CAFFEINE-TREATED RAT MYOCARDIUM

Skinned fiber preparations are used to obtain the maximal contractile activation of isolated myocardial preparations. Tetanic contractions e licited in the presence of sarcoplasmic reticulum inhibitors have also been used as an alternative method to produce maximal active tension in the intact myocardium. In this work our purpose was to define the b est conditions to obtain tetanic contractions in the rat myocardium an d to compare the influence of muscle length and inotropic intervention s (Ca2+ and Bay K 8644) in the tension produced in twitches and tetani c contractures. Papillary muscles were mounted in a perfusion chamber to record isometric force. Tetanic contractions were elicited by using suprathreshold stimulation with rectangular pulses (10 ms duration) a t 5 Hz in the presence of 2.5 mM caffeine. Caffeine depressed the twit ch tension but the tetanic tension was similar to that produced under steady-state stimulation (0.5 Hz) in control conditions. Tetanic and t witch tensions were similar along the whole extension of the length-te nsion curve and under the positive inotropic effects produced by Ca2(0.25 to 3.75 mM) or by the Ca2+-channel agonist Bay K 8644 (1 mu M). During long tetanic stimuli (60 s) a time-dependent tension decay was observed. This decay was prolonged by reducing the extracellular K+ fr om 5.4 to 1.0 mM, suggesting that Ca2+ extrusion through the Na-Ca exc hanger seems to occur during tetanic stimulation. Since tetanic tensio n was never higher than the tension obtained in twitches elicited at t he same Ca2+ concentration (0.5 Hz), we conclude that tetanic contract ures represent a useful tool to investigate the contractile response o f intact myocardial preparations with a nonfunctional sarcoplasmic ret iculum. However, our results indicate that the maximal activation of t he contractile machinery in the rat myocardium seems to be only achiev ed in twitches elicited in high extracellular Ca2+ concentrations and with a functional sarcoplasmic reticulum.