Enhancement of murine cardiac chronotropy by the molecular transfer of the human β2 adrenergic receptor cDNA

Cardiac pacemaking offers a unique opportunity for direct gene transfer into the heart. An experimental system was developed to assay the effects 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 with isolated myocytes, and transplanted as well as endogenous murine hearts. Murine embryonic cardiac myocytes were transiently transfected with plasmid constructs. The total percentage of myocytes spontaneously contracting was greater in beta(2)AR transfected cells, as compared with control cells (67 vs. 42 +/- 5%). In addition, the percentage of myocytes 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 transfected myocytes at baseline (71+/-14 vs. 50+/-10 bpm; P < 0.001) as well as with the addition of 10(-3) M isoproterenol (98+/-26 vs. 75+/-18 bpm; P < 0.05). Based on these results, a murine neonatal cardiac transplantation model was used to study the ex vivo effects of targeted expression of beta(2)AR. The constructs were transfected into the right 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 injection into the right atrium of the endogenous heart, These results were similar to the ex vivo data with injection of the beta(2)AR constructs increasing the endogenous heart rates by similar to 40%, as compared with control injected hearts (550+/-42 vs. 390+/-37 bpm; P < 0.05). These studies demonstrate that local targeting of gene expression may be a feasible modality to regulate the cardiac pacemaking activity.