1. Left ventricular (LV) diastolic function was investigated in three different age groups (15, 28 and 50 weeks) of paired spontaneously hypertensive (SHR) and normotensive (WKY) rats under pentobarbital anaesthesia. A time constant of LV pressure decay, represented by T, was used as an index of LV relaxation. We assessed the relationship between haemodynamic parameters and LV structural components as quantified by microspectrophotometry (MSP), using multivariate analysis.

2. T was significantly prolonged in the 28 and 50 week old SHR compared with their normotensive counterparts (P < 0.05 and P < 0.01, respectively). T was prolonged by volume loading but was not affected with afterload elevation by angiotensin infusion in all age groups of the SHR and WKY.

3. LV wall thickness was greater in the SHR at all ages and was positively correlated with T (r = 0.42, P < 0.05).

4. A significant correlation was found between the increase in cardiac muscle fibre and collagen, the decrease in elastin and glycoprotein, and T on multivariate analysis (r = 0.53, P < 0.05).

5. We conclude that LV relaxation of SHR is disturbed from a relatively young age (28 weeks), for which we consider myocardial hypertrophy and LV structural changes found by MSP as being responsible.

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