ENHANCEMENT OF MURINE CARDIAC CHRONOTROPY BY THE MOLECULAR TRANSFER OF THE HUMAN BETA(2) ADRENERGIC-RECEPTOR CDNA

Cardiac pacemaking offers a unique opportunity for direct gene transfe r into the heart. An experimental system was developed to assay the ef fects of transferring the human beta(2) adrenergic receptor (beta(2)AR ) under in vitro, ex vivo, and finally in vivo conditions. Constructs encoding either beta(2)AR or LacZ were used in chronotropy studies wit h isolated myocytes, and transplanted as well as endogenous murine hea rts. Murine embryonic cardiac myocytes were transiently transfected wi th plasmid constructs. The total percentage of myocytes spontaneously contracting was greater in beta(2)AR transfected cells, as compared wi th control cells (67 vs. 42 +/- 5%). In addition, the percentage of my ocytes with chronotropic rates > 60 beats per minute (bpm) was higher in the beta(2)AR population, as compared with control cells (37 vs. 15 +/-5%). The average contractile rate was greater in the beta(2)AR tran sfected myocytes at baseline (71+/-14 vs. 50+/-10 bpm; P < 0.001) as w ell as with the addition of 10(-3) M isoproterenol (98+/-26 vs. 75+/-1 8 bpm; P < 0.05). Based on these results, a murine neonatal cardiac tr ansplantation model was used to study the ex vivo effects of targeted expression of beta(2)AR. The constructs were transfected into the righ t atrium of transplanted hearts. Injection of the beta(2)AR construct increased the heart rate by similar to 40% (224+/-37 vs. 161+/-42 bpm; P < 0.005). Finally, the constructs were tested in vivo with injectio n into the right atrium of the endogenous heart, These results were si milar to the ex vivo data with injection of the beta(2)AR constructs i ncreasing the endogenous heart rates by similar to 40%, as compared wi th control injected hearts (550+/-42 vs. 390+/-37 bpm; P < 0.05). Thes e studies demonstrate that local targeting of gene expression may be a feasible modality to regulate the cardiac pacemaking activity.