Chronic activation of extracellular-signal-regulated protein kinases by phenylephrine is required to elicit a hypertrophic response in cardiac myocytes

Extracellular-signal-regulated protein kinases (ERKs) are activated rapidly and transiently in response to phenylephrine (PE) and endothelin-1 (ET-1) in cardiac myocytes, but whether this is linked to the subsequent development of the hypertrophic phenotype remains equivocal. To investigate this, we examined the dependence of the hypertrophic response on the length of exposure to PE in neonatal myocyte cultures. In addition to the initial transient activation of ERKs (maximum at 5—10min), PE (10μM) induced a second, more prolonged peak of activity several hours later. The activity of a transfected atrial natriuretic factor—luciferase reporter gene was increased 10- to 24-fold by PE. This response was inhibited by the α₁-antagonist prazosin (100nM) and by U0126 (10μM) and PD184352 (1μM), inhibitors of ERK activation, irrespective of whether these were added before or up to 24h after the addition of PE. Prazosin had no effect on ET-1 (50nM)-stimulated atrial natriuretic factor—luciferase activity. Protein synthesis was enhanced by 35±6% by PE, and this was blocked by prazosin added 1h after the addition of PE, but decreased only by half when added 8h after PE. Similarly, PE (48h) increased myocyte area by 49% and this was prevented by prazosin added 1h after PE, but decreased only by half when added at 24h. These results demonstrate that prolonged exposure to PE is required to elicit alterations in gene expression, protein synthesis and cell size, characteristic of hypertrophied myocytes, and they confirm that the initial peak of ERK activity is insufficient to trigger hypertrophic responses.