INOSITOL PHOSPHATE RELEASE AND METABOLISM IN RAT LEFT ATRIA

The phosphatidylinositol (PtdIns) turnover pathway in intact heart tissue differs from that in most cell types in that products of the inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3] kinase pathway are not detected in H-3-labeling studies. In contrast, Ins(1,4,5)P-3 kinase products are detected in isolated neonatal cardiomyocytes. To understand the basis for the observed properties of the cardiac pathway, a detailed study of inositol phosphate (InsP) release has been undertaken by using isolated adult rat left atria. Addition of norepinephrine to H-3-labeled atria caused a slow increase in H-3-labeled Ins(1,4,5)P-3 and a more rapid increase in H-3-labeled Ins(1,4)P-2, its immediate dephosphorylation product. The mass of Ins(1,4,5)P-3 was high in unstimulated atria (13.5+/-1.1 pmol/mg tissue, mean+/-SEM, n=4) and did not change with stimulation. Measurements of the specific activities of Ins(1,4,5)P-3 and PtdIns(4,5)P-2 provided an estimate of the turnover rate of Ins(1,4,5)P-3 that was 20- to 40-fold lower than the rate of accumulation of H-3 label in InsP(1) and InsP(2). In agreement with this, specific activities of InsP(1) and InsP(2) were higher than the specific activity of InsP(3) in both control and stimulated atria. Neomycin (5 mmol/L) did not inhibit the accumulation of H-3-labeled InsP(1) and InsP(2) in left atria, even though it reduced the accumulation of H-3 label in Ins(1,4,5)P-3, providing evidence that InsP(1) and InsP(2) do not derive primarily from Ins(1,4,5)P-3. Stimulation with norepinephrine for 20 minutes resulted in a parallel decrease in H-3-labeled Ins(1,4,5)P-3 and in Ins(1,4,5)P-3 mass, demonstrating that atria do not contain two different pools of Ins(1,4,5)P-3. In contrast with findings in atria, the mass content of Ins(1,4,5)P-3 was low in unstimulated cardiomyocytes and increased with norepinephrine stimulation. Furthermore, neomycin (5 mmol/L) inhibited the accumulation of H-3 label in all of the InsPs, demonstrating that they are derived from Ins(1,4,5)P-3. The data obtained when using left atria cannot be explained by models proposing either receptor-stimulated breakdown of Ins(1,4,5)P-3 or the presence of more than one pool of Ins(1,4,5)P-3 with different specific activities and are most readily explained by a model in which stimulation of receptors causes primarily the hydrolysis of PtdIns(4)P to Ins(1,4)P-2. The high and unchanging content of Ins(1,4,5)P-3 may be related, in some way, to its poor Ca2+-mobilizing activity in heart tissue.