Effects of microinjection of picrotoxin into posterior hypothalamus on ventricular electric stability

Ventricular fibrillation threshold (VFT), serum potassium, and monophasic action potentials (MAP) have been assessed before and after microinjection of picrotoxin (Pic) into posterior hypothalamus in rabbits. Pic (2 and 3 micrograms) brought about a biphasic effect on VFT, an initial decrease followed by a notable increase. Pretreatment with spinal cord transection almost abolished the descending phase, whereas pretreatment with vagotomy depressed the ascending phase significantly. Following the microinjection of Pic, serum potassium rose up to 5.0 mmol.L-1 at 30 min. The amplitude of MAP (MAPA) and Vmax were lessened in rabbits, while in vagotomized rabbits, the changes of MAP were aggravated, and the duration of MAP was shortened. Phentolamine counteracted the changes of MAP induced by Pic. These results suggested that the initial decrease of VFT caused by Pic derived mainly from the direct action and myocardial ischemia of sympathetic activation, while its subsequent elevation was caused by vagal activation and hyperkalemia.