VENTRICULAR OVERDRIVE PACING-INDUCED ANTIISCHEMIC EFFECT - A CONSCIOUS RABBIT MODEL OF PRECONDITIONING

To study whether ventricular overdrive pacing (VOP) induces preconditi oning, rabbits were equipped with right ventricular electrode catheter s for pacing and intracavital recording and polyethylene cannulas in t he left ventricle and right carotid artery to measure intraventricular pressure and blood pressure. One week after surgery in conscious anim als, VOP at 500 beats/min over 2, 5, or 10 min resulted in an intracav ital S-T segment elevation, shortening of ventricular effective refrac tory period, decrease in maximum rate of pressure development and bloo d pressure, and increase in left ventricular end-diastolic pressure pr oportional to the duration of stimulation. A 5-min preconditioning VOP applied 5 or 30 min before a 10-min VOP markedly attenuated ischemic changes, whereas a 2-min VOP had no effect. In anesthetized rabbits, a 5-min VOP slightly increased guanosine 3',5'-cyclic monophosphate (cG MP) and profoundly elevated adenosine 3',5'-cyclic monophosphate (cAMP ) content in left ventricular samples. When this VOP was preceded (5 o r 30 min) by a preconditioning VOP, the cAMP increase was significantl y attenuated, whereas the cGMP increase was amplified. We conclude tha t a single 5-min VOP induces preconditioning in association with alter ations in cardiac cyclic nucleotide contents.