HYPOXIA INVERTS THE NEGATIVE CHRONOTROPIC RESPONSE TO NOREPINEPHRINE IN NORMOXIA IN CULTURED NEONATAL RAT CARDIAC MYOCYTES - ROLE OF THE ALPHA(1)-ADRENERGIC SIGNAL-TRANSDUCTION SYSTEM

In the present study, we investigated the effect of hypoxia on the chr onotropic response to norepinephrine (NE) of cultured neonatal rat ven tricular myocytes. We measured beating of myocytes with the Fotonic se nsor(TM), using a newly developed method for a noncontact displacement measurement. The beating rate counted with the sensor had a high corr elation coefficient with that counted visually under a microscope (r = 0.997, P < 0.01). NE concentrations of 10(-8)-10(-4) M caused negativ e chronotropy dose dependently in the presence of 5 x 10(-7) M propran olol. NE-induced chronotropy was completely antagonized by 10(-6) M pr azosin. Three hours hypoxia decreased the spontaneous beating rate 40% (P < 0.01). Negative chronotropy induced by 10(-4) M NE in normoxia w as inverted to positive and was antagonized by prazosin. Hypoxia incre ased the basal level of inositol 1,4,5-trisphosphate (Ins(1,4,5)P-3) t o 190% (P < 0.01), while NE-stimulated Ins(1,4,5)P-3 production was si gnificantly suppressed. Immunoblotting analysis of G protein subunits demonstrated no quantitative changes in Gi alpha, Gq alpha, Go alpha a nd G(beta common) subunits in hypoxia. In a saturation binding assay w ith [H-3]prazosin, K-d values were increased to 152% by hypoxia (P < 0 .05) without significant change in B-max. Basal activity of low K-m-GT Pase was increased to 122% by hypoxia (P < 0.05). These results sugges t that the hypoxia-induced increase in low-K-m GTPase activity, which could stimulate phospholipase C by an activated alpha(GTP) subunit of G protein and consequently induce receptor-independent increase in Ins (1,4,5)P-3, may be responsible for the inversion of the NE-induced neg ative chronotropic response in normoxia.