Effects of norepinephrine and cardiotrophin-1 on phospholipase D activity and incorporation of myristic acid into phosphatidylcholine in rat heart.

The present study is part of a project on phospholipase D (PLD) in cardiac hypertrophy and analyzed effects on PLD activity of two growth stimuli, norepinephrine (NE) and cardiotrophin-1 (CT-1), in incubated rat heart. Phosphatidylcholine (PC) was labeled by (3)H-myristic acid. PLD produced (3)H-phosphatidylethanol ((3)H-PEth) from (3)H-PC in the presence of ethanol and maintained a basal formation of (3)H-PEth. Short-term and long-term exposure to NE for 2 or 13 h, respectively, enhanced the formation of (3)H-PEth, which was blocked by prazosin. Long-term pretreatment with NE or CT-1 increased the incorporation of (3)H-myristic acid into PC, which was blocked by atenolol. When the (3)H-PEth formation was expressed as a fraction of (3)H-PC, PLD activity seemingly was unchanged (NE) or markedly reduced (CT-1); the true effects, namely, stimulation by NE and nonresponsiveness towards CT-1, were unraveled by atenolol (NE) or when PLD activity was expressed as (3)H-PEth per ng protein. In conclusion, alpha-adrenoceptor activation increased PLD activity. Long-term treatment with NE (via beta-receptors) or CT-1 enhanced the (3)H-myristic acid incorporation into a PC compartment, that was not available for the alpha-receptor-mediated PLD activation. These results were discussed in regard to cellular mechanisms of cardiac hypertrophy and to the transphosphatidylation assay of PLD.