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Inhibition of the formation or action of angiotensin II reverses attenuated K⁺ currents in type 1 and type 2 diabetes

Yakhin Shimoni

1. Transient and sustained calcium-independent outward K⁺ currents (I_t and I_SS) as well as action potentials were recorded in cardiac ventricular myocytes isolated from two models of diabetes mellitus.
2. Rats injected (I.V.) with streptozotocin (STZ, 100 mg kg^-1) 6-10 days before cell isolation developed insulin-dependent (type 1) diabetes. I_t and I_SS were attenuated and the action potential prolonged. Incubation of myocytes (6-9 h) with the angiotensin II (ATII) receptor blockers saralasin or valsartan (1 µM) significantly augmented these currents. Inclusion of valsartan (1 g l^-1) in the drinking water for 5-10 days prior to and following STZ injection partially prevented current attenuation.
3. Incubation of myocytes from STZ-treated rats (6-9 h) with 1 µM quinapril, an angiotensin-converting enzyme (ACE) inhibitor, significantly augmented I_t and I_SS and shortened the ventricular action potential. I_t augmentation was not due to changes in steady-state inactivation or in recovery from inactivation. No acute effects of quinapril were observed.
4. The effects of quinapril and valsartan were abolished by 2 µM cycloheximide.
5. Myocytes were isolated from the db/db mouse, a leptin receptor mutant that develops symptoms of non-insulin-dependent (type 2) diabetes. K⁺ currents in these cells were also attenuated, and the action potentials prolonged. Incubation of these cells (> 6 h) with valsartan (1 µM) significantly enhanced the transient and sustained outward currents.
6. These results confirm recent suggestions that cardiac myocytes contain a renin-angiotensin system, which is activated in diabetes. It is proposed that chronic release of ATII leads to changes in ionic currents and action potentials, which can be reversed by blocking the formation or action of ATII. This may underlie the proven benefits of ATII receptor blockade or ACE inhibition in diabetes, by providing protection against cardiac arrhythmias.

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