Abstract-The possible role of calcineurin in the attenuation of cardiac hypertrophy and fibrosis by blockade of the angiotensin II type 1 (AT 1 ) receptor was investigated in Dahl salt-sensitive (DS) rats. The effect of the calcineurin inhibitor FK506 was also studied. DS rats progressively developed severe hypertension when fed a diet containing 8% NaCl from 7 weeks of age. In addition, marked cardiac hypertrophy and fibrosis were apparent and the activity of calcineurin and its mRNA expression in the myocardium was increased in these animals at 12 weeks in comparison with age-matched Dahl salt-resistant rats. The abundance of angiotensin-converting enzyme (ACE) and transforming growth factor (TGF)-1 mRNAs was also increased in the hearts of DS rats at 12 weeks. Treatment of DS rats with a non-antihypertensive dose of the selective AT 1 receptor blocker candesartan (1 mg/kg per day) or FK506 (0.1 mg/kg per day) from 7 to 12 weeks attenuated both calcineurin activity and its mRNA expression in the heart, as well as the development of cardiac hypertrophy and fibrosis, without affecting cardiac function. There are at least 2 isoforms for Ang II receptors, which are designated as AT 1 and AT 2 , and the AT 1 receptor is further subdivided into AT 1A and AT 1B . It is generally accepted that most of the traditional Ang II functions in the cardiovascular system are attributable to the AT 1 receptor. 3 Angiotensin-converting enzyme (ACE) inhibitors or AT 1 receptor blockers induce the regression or prevent the development of left ventricular (LV) hypertrophy, both in animal models 4 -6 and in hypertensive patients. 7,8 However, it has proved difficult to determine whether the antagonistic effects of ACE inhibitors and AT 1 receptor blockers on Ang II-induced growth promotion or the concomitant systemic hemodynamic effects of these agents underlie their beneficial action with regard to this condition. A non-antihypertensive dose of an ACE inhibitor was shown to reverse LV hypertrophy in aortic banded rats, 9,10 and the antihypertrophic effect of an AT 1 receptor blocker was shown to be greater than that of hydralazine, despite the greater antihypertensive effect of hydralazine, in spontaneously hypertensive rats. 6 These observations suggest that blood pressure reduction alone is not sufficient to prevent target organ damage and that the additional control of local or neurohumoral factors might also be required.An intracellular signaling pathway that includes the Ca 2ϩ -dependent protein phosphatase calcineurin has been shown to underlie cardiac hypertrophy. 11 Calcineurin has also been shown to play a key role in the development of pressure overload-induced cardiac hypertrophy. 12,13 A recent study suggested that calcineurin is involved in the development of cardiac hypertrophy induced by mineralocorticoid excess. 14 Furthermore, treatment of cultured cardiac myocytes with Ang II or phenylephrine results in activation of calcineurin. 15 However, the effect of the cardiac renin-angiotensin system (RAS) on calcineurin ...