Direct effects of the angiotensin-converting enzyme inhibitor ramiprilat on adult rat ventricular cardiomyocytes

AIM

Angiotensin-converting enzyme (ACE) inhibitors like ramiprilat bind to ACE expressed on the cell surface of endothelial cells and induce cell-specific signalling including the activation of activator protein (AP)-1. The present study addressed the question whether ramiprilat exerts a similar effect on adult ventricular cardiomyocytes, i.e. activates the AP-1 or modifies contractile performance. It was further aimed to decide whether such effects depend on bradykinin receptors or whether they are directly mediated via ACE.

METHODS

Adult rat ventricular cardiomyocytes were isolated and cultured. mRNA expression of ACE was investigated by RT-PCR, AP-1 activation by gel mobility shift assays, and cardiac contractile performance by electrical pacing of isolated cells and analysis of cell shortening via a line-camera.

RESULTS

Cardiomyocytes stably express ACE. Ramiprilat increased maximal contraction velocity and shortened the time-to-peak of contraction. In contrast to effects evoked by bradykinin, such effects caused by ramiprilat were not attenuated by HOE 140, a bradykinin-receptor antagonist. These effects were also not attenuated in the presence of l-nitro-arginine, used to mimic bradykinin-dependent signalling. In cardiomyocytes, bradykinin but not ramiprilat activated AP-1. Ramiprilat activates AP-1 in endothelial cells that are known to respond to ramiprilat in this way.

CONCLUSION

Ramiprilat exerts direct, bradykinin-receptor independent effects on cardiomyocytes that improve cellular function without a corresponding effect on AP-1 activation or induction of AP-1 dependent effects. This newly described effect of ramiprilat may contribute to the protective effects seen by application of ACE inhibitors.

Transcription activator protein 1 mediates alpha- but not beta-adrenergic hypertrophic growth responses in adult cardiomyocytes

In some models of cardiac hypertrophy, activation of activator protein 1 (AP-1) correlates with growth. However, AP-1 is also activated by stimuli not involved in cardiac growth. This raises the following questions: does AP-1 plays a causal role for cardiomyocyte growth, and is this role model or stimulus dependent? We used a single model to address these questions, i.e., ventricular cardiomyocytes of adult rats, and two growth stimuli, i.e., alpha- and beta-adrenoceptor agonists [10 microM phenylephrine (PE) and 1 microM isoprenaline (Iso), respectively]. After 1 h of stimulation with PE, mRNA expression of c-Fos and c-Jun was upregulated to 185 +/- 32 and 132 +/- 13% of control. Fos and Jun proteins formed the AP-1 complex. PE stimulated DNA binding activity of AP-1 to 165 +/- 22% of control within 2 h and increased protein synthesis to 161 +/- 27% of control and cross-sectional area to 126 +/- 4% of control. Inhibition of AP-1 binding activity by cAMP response element (CRE) decoy oligonucleotides abolished both of these growth responses. Iso stimulated AP-1 binding activity to 203 +/- 19% of control within 2 h and stimulated protein synthesis to 145 +/- 17% of control. However, the growth effect of Iso was not abolished by CRE decoys: Iso increased protein synthesis to 158 +/- 17% of control in the presence of CRE. In conclusion, AP-1 is a causal mediator of the alpha-adrenergic, but not the beta-adrenergic, growth response of cardiomyocytes.