Developmental Changes in Guanine Nucleotide Regulatory Proteins in the Rat Myocardial Alpha 1-adrenergic Receptor Complex

During development, the cardiac alpha 1-adrenergic chronotropic response changes from positive in the neonate to negative in the adult. The negative chronotropic effect of alpha 1-adrenergic stimulation in the adult depends on maturation of sympathetic innervation and the presence of a pertussis toxin (PT)-sensitive guanine nucleotide-binding (G) protein. To examine the possibility of a developmental change in coupling of a PT-sensitive G protein to the alpha 1-adrenergic receptor, radioligand binding experiments with the iodinated alpha 1-selective radioligand [125I]-I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([ 125I]-IBE 2254) were performed on membranes prepared from control and PT-treated neonatal and adult rat hearts. Scatchard analysis showed fewer alpha 1-adrenergic receptors in the adult than in the neonate (168 +/- 10 fmol/mg protein in the neonate vs. 124 +/- 13 fmol/mg protein in the adult), but similar affinities (equilibrium dissociation constant 124 +/- 29 pM in the neonate vs. 140 +/- 34 pM in the adult). PT treatment did not alter the results. In both the neonate and adult, 5'-guanylylimidodiphosphate [Gpp(NH)p, 500 microM] shifted the l-epinephrine competition curve to the right and increased the slope factor toward unity. PT had no effect on the l-epinephrine competition curve in the neonate. However, in the adult PT itself caused a partial shift in the agonist competition curve, reducing but not eliminating the effect of Gpp(NH)p. Consistent with the results from the binding experiments, PT did not have any effect on the alpha 1-adrenergic-mediated positive chronotropic response in the neonate, whereas in the adult the alpha 1-adrenergic-mediated negative chronotropic response was completely converted to a positive one after PT treatment. These results indicate the presence of a PT-insensitive G protein in the neonatal and adult rat heart and the acquisition of a PT-sensitive G protein linked to the negative chronotropic response during development.