Inositol phosphate release and metabolism in rat left atria
Woodcock, E.A.; Suss, M.B.; Anderson, K.E.
Circulation Research 76(2): 252-260
1995
ISSN/ISBN: 0009-7330 PMID: 7834836 Document Number: 439972
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 3H-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 3H-labeled atria caused a slow increase in 3H-labeled Ins(1,4,5)P-3 and a more rapid increase in 3H-labeled Ins(1,4)P-3, 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 3H label in InsP-1 and InsP-2. In agreement with this, specific activities of InsP-2 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 3H-labeled InsP-1 and InsP-2 in left atria, even though it reduced the accumulation of 3H 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 3H-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 3H 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 Ca-2+-mobilizing activity in heart tissue.