Guanosine metabolism in adult rat cardiac myocytes: ribose-enhanced GTP synthesis from extracellular guanosine.

The metabolic fate of transported guanosine was examined in adult rat cardiac myocytes. Freshly isolated cells were incubated with 10 microM or 100 microM [3H]guanosine and the nucleotide products extracted and examined for radiolabel distribution. The data presented show significant incorporation of guanosine into the 5'-nucleotide pool, and a marked stimulation of that incorporation by ribose. An average of 233 pmol/mg cell protein extracellular guanosine was incorporated into the cellular 5'-nucleotides over 90 min at both 10 microM and 100 microM external nucleoside. This appeared primarily as GTP (approx. 204 pmol/mg cell protein in 90 min). Only guanine nucleotides contained radiolabel; adenine nucleotides and IMP remained unlabelled even after 90 min incubation of the cells with [3H]guanosine. Addition of 5 mM ribose to the medium stimulated guanosine incorporation into 5'-nucleotides 1.6-fold (380 pmol/mg protein vs 234 pmol/mg over 90 min at 10 microM guanosine), but did not enhance the amount of guanosine transported into the cells. Intracellular guanosine concentrations exceeded those of the incubation medium at both external guanosine concentrations studied. More [3H]guanosine was salvaged at 100 microM than at 10 microM external guanosine (562 vs 380 pmol/mg protein in 90 min), but only if ribose was present in the medium. We conclude from these studies that guanosine is salvaged by heart muscle, and that at high guanosine levels the rate of guanosine salvage appears dependent on the availability of phosphoribosylpyrophosphate within the cells. At lower guanosine levels in the presence of ribose, cell guanine concentrations limit the rate of guanosine incorporation into 5'-nucleotides.