Abstract

Chronic stimulation of beta-adrenoceptors leads to increased mRNA and protein levels of pertussis toxin (PTX)-sensitive guanine nucleotide-binding proteins (G proteins) in the heart. In the present study the time course is reported of the effect of isoprenaline infusions on myocardial mRNA levels of Gi alpha-2, Gi alpha-3, G(o) alpha, Gs alpha, and G beta and myocardial levels of PTX-sensitive G proteins. Rats were treated by subcutaneous infusions, with osmotic minipumps, of 0.9% NaCl, isoprenaline (2.4 mg/kg/day), propranolol (9.9 mg/kg/day), or a combination thereof for 1, 2, 3, 4, or 8 days, and two groups were treated with NaCl or isoprenaline for 13 or 26 days. Additional groups of rats were treated with 0.24 or 0.07 mg/kg/day for 4 days to determine the dose dependency of the effects of isoprenaline. mRNA concentrations were determined by standardized slot blotting with 32P-labeled cDNA or cRNA probes. In isoprenaline-treated rats, mRNA levels of all members of the Gi alpha/G(o) alpha family increased gradually in parallel. The increase in Gi alpha-2, Gi alpha-3, and G(o) alpha mRNA levels reached significance on days 2-3, reached maximal values on days 3-4, and remained stable over the total time of treatment of up to 26 days. Compared with NaCl, maximal increases were 77% (Gi alpha-2; 16.4 versus 9.3 pg/micrograms), 58% (Gi alpha-3; 4.65 versus 2.95 pg/micrograms), and 78% (G(o) alpha; 0.40 versus 0.22 pg/microgram). Gs alpha mRNA levels (about 30 pg/micrograms) and G beta mRNA levels remained unchanged. In isoprenaline-treated rats myocardial levels of PTX-sensitive G proteins increased by maximally 54%, compared with control. The time course differed slightly from the time course of mRNA up-regulation in the first 3 days of treatment and paralleled the increase in mRNA levels from day 4 on. Propranolol had no effect on G alpha mRNA or protein levels when given alone but abolished all effects of isoprenaline when given in combination. Isoprenaline at a dose of 0.24 mg/kg/day still induced an increase in Gi alpha-2 mRNA levels of 24%, without any effects on histopathology of the myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)