The levels of adrenomedullin (ADM), a newly discovered vasodilating and natriuretic peptide, are elevated in plasma and ventricular myocardium in human congestive heart failure suggesting that cardiac synthesis may contribute to the plasma concentrations of ADM. To examine the time course of induction and mechanisms regulating cardiac ADM gene expression, we determined the effect of acute and short-term cardiac overload on ventricular ADM mRNA and immunoreactive ADM (ir-ADM) levels in conscious rats. Acute pressure overload was produced by infusion of arginine-vasopressin (AVP, 0.05 μg/kg/min, iv) for 2 h into 12-week-old hypertensive TGR (mREN-2)27 rats and normotensive Sprague-Dawley (SD) rats. Hypertension and marked left ventricular hypertrophy were associated with 2,2-times higher ir-ADM levels in the left ventricular epicardial layer (178 ± 36 vs. 81 ± 23 fmol/g, P<0.05) and 2.6-times higher ir-ADM levels in the left ventricular endocardial layer (213 ±23 vs. 83 ±22 fmol/g. P<0.01). The infusion of AVP for 2 h in normotensive rats produced rapid increases in the levels of left ventricular ADM mRNA (epicardial layer: 1.6-fold, P<0.05) and ir-ADM (endocardial layer: from 83 ± 22 to 140 ± 12 fmol/g, P<0.05), whereas ventricular ADM mRNA and ir-ADM levels did not change significantly in hypertensive rats. Short-term cardiac overload, induced by administration o[ angiotensin II (33.3 μg/kg/h, sc, osmotic minipumps) for two weeks in normotensive SD rats resulted in left ventricular hypertrophy (3.05 ± 0.17 vs 2.75 ± 0.3 mg/g, P < 0.05) and a 1.5-fold increase (P<0.05) in ventricular ADM mRNA levels. In conclusion, the present results show that pressure overload acutely stimulated ventricular ADM gone expression in conscious normotensive rats suggesting a potential beneficial role for endogenous ADM production in the heart against cardiac overload. Since pressure overload-induced increase in ADM synthesis was attenuated in hypertensive rats, alterations in the ADM system rally contribute to the pathogenesis of hypertension in the TGR(mREN-2)27 rat.
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