QUANTIFICATION OF THE VOLTAGE-RESPONSE RELATIONSHIP BETWEEN PUNCTATE AND FIELD ELECTRICAL-STIMULATION AND THE FUNCTION OF ISOLATED RAT LEFTATRIA AND PAPILLARY-MUSCLES

The effects of varying the electrical stimulation voltage on the norad renergic tone and systolic and diastolic cardiac function of isolated rat cardiac preparations is unknown. If a wide range of voltages subst antially alters the basal noradrerengic tone of isolated cardiac prepa rations, highly variable responses may complicate the interpretation o f the cardiac effects of interventions, such as drugs or toxins. This study was designed to determine whether field or punctate electrode st imulation altered systolic and diastolic cardiac function of isolated rat atrial and ventricular muscle preparations through a beta-adrenerg ic receptor-mediated mechanism. Isolated left atria and right ventricu lar papillary muscles from adult Sprague-Dawley rats were stimulated t o contract isometrically in Krebs bicarbonate buffer (30 degrees C, pH 7.4, 0.5 Hz). Cardiac muscle function was assessed by determining car diac contractility (dF/dt), relaxation (80% relaxation time), and musc le stiffness (changes in resting force). Muscles were electrically sti mulated to contract using either punctate or field electrodes. Basal v alues of cardiac function were determined at threshold voltages, and v oltage was increased in stepwise fashion, over a wide range, to obtain a voltage response-relation to cardiac function. In left atrial prepa rations, both punctate and field electrical stimulation caused a 200% increase in cardiac contractility. The greatest changes in contractili ty occurred at near threshold voltages (less than 5 volts for punctate and 19 volts for field stimulation). The voltage-dependent increases in cardiac contractility were attenuated or abolished by pretreatment with atenolol (10 mu mol/L; selective beta(1)-antagonist) or reserpine (5 mg/kg i.p., 24 hr before euthanasia). In contrast to left atrial p reparations, neither field or punctate electrical stimulation had any effect on the cardiac contractility of papillary muscle preparations. Neither field or punctate electrical stimulation had any effect on car diac relaxation (80% relaxation time) or muscle stiffness (changes in resting force) of left atrial or papillary muscle preparations. This s tudy demonstrates that punctate and field electrical stimulation can s ignificantly increase cardiac contractility of rat left atrial prepara tions at near threshold voltages by increasing noradrenergic tone. Thu s, control of the voltage range may be required to reduce variation in cardiac function of isolated rat left atrial preparations.