Impaired relaxation of the hypertrophied myocardium is potentiated by angiotensin II.

Relaxation delay is an important feature of hypertensive heart disease which impairs diastolic coronary flow and ventricular filling and therefore contributes to heart failure. We investigated the hypothesis that impaired relaxation is a property of the myocardium, rather than the consequence of ischaemia or interstitial fibrosis. A new videomicroscope system was used to define the contraction-relaxation cycle of isolated cardiac myocytes from spontaneously hypertensive rats (SHR) and normotensive control (Wistar-Kyoto, WKY) rats. The SHR cells showed a marked relaxation delay. Angiotensin II (Ang II) increased the contraction maximum by about 35% in WKY rats and induced a relaxation delay. In SHR Ang II greatly potentiated this relaxation delay. Our results demonstrate that impairment of relaxation is a property of the single cardiomyocyte. Angiotensin II induces a relaxation delay that is independent of blood pressure. The combination of hypertrophy and high levels of Ang II potentiates relaxation impairment and may therefore contribute to hypertensive left ventricular failure.