Combined renin-angiotensin system blockade and dietary sodium restriction impairs cardiomyocyte contractility

Blockade of the renin-angiotensin system (RAS) by combined angiotensin-converting enzyme inhibitor and angiotensin type 1 receptor (AT(1)) antagonist treatment with reduced dietary sodium intake produces suppression of cardiac growth and regression of cardiac hypertrophy. The purpose of this study was to investigate whether cardiac growth suppression by combined RAS blockade under conditions of dietary sodium restriction is associated with cardiomyocyte atrophy and contractile dysfunction and whether this intervention modifies cardiomyocyte inotropic responsiveness to angiotensin II (Ang II).

Rats were fed a high (4% w/w) or low (0.2% w/w) NaCl diet for six days. Both groups were then given a combined intraperitoneal injection of perindopril (6 mg/kg/day) and losartan (10 mg/kg/day) with maintained dietary treatment for another seven days. At the end of the treatment period, animals were anaesthetised and their hearts were removed and weighed. Left ventricular cardiomyocytes were isolated by enzymatic dissociation and cell dimensions were evaluated. A line scan camera and digital imaging technique were used to assess cardiomyocyte contraction and inotropic responses to exogenous Ang II (10 to 10(-8) M).

Dietary treatment alone had no effect on body growth, whereas combined RAS blockade suppressed somatic growth in the low sodium (LS) group, compared with the high sodium (HS) group. This growth suppression in the LS group was also evident in the heart at the organ and cellular level. Studies of cardiomyocyte contraction showed that myocytes from the LS group exhibited contractile instability and depression of contractile performance. Compared with the HS group, myocytes from the LS group showed a significant reduction in maximum cell shortening (6.40+0.17 vs. 7.32+0.16% resting length, p<0.05), and maximum rate of shortening (3.85+0.14 vs. 4.29+0.11 cell length/ms, p<0.05). Myocytes of the HS group exhibited negative inotropic responses to Ang II at all concentrations tested, with a significant reduction in maximum cell shortening by 11 16% after 12 minutes peptide exposure (p<0.05 vs. non-treated control). In comparison, Ang II elicited both positive and negative responses in myocytes from the LS group, with a predominant negative inotropic effect.

This study provides evidence that combined RAS blockade treatment under restricted sodium intake conditions can impair cardiomyocyte contractile function in association with cardiomyocyte growth suppression. Chronic RAS blockade qualitatively alters the intrinsic inotropic status and responsiveness of ventricular cardiomyocytes, and this shift is further modulated by dietary sodium intake conditions.