Angiotensin II type 1 receptor mediates partially hyposmotic-induced increase of I Ks current in guinea pig atrium

Losartan inhibits hyposmotic-induced increase of IKs current and shortening of action potential duration in guinea pig atrial myocytes

Objective:

The present study aims to investigate the effect of losartan, an selective angiotensin II type 1 receptor (AT₁R) blocker, on both the increase of I_Ks current and shortening of action potential duration (APD) induced by stretch of atrial myocytes, and to uncover the mechanism underlying the treatment of fibrillation (AF) by AT₁R blockers.

Methods:

Hyposmotic solution (Hypo-S) was applied in the guinea pig atrial myocytes to simulate cell stretch, then patch-clamp technique was applied to record the I_Ks and APD in atrial myocytes.

Results:

Hypo-S increased the I_Ks by 105.6%, while Hypo-S+1-20 µM of losartan only increased the I_Ks by 70.3-75.5% (p<0.05 vs. Hypo-S). Meanwhile, Hypo-S shortened APD₉₀ by 20.2%, while Hypo-S+1-20 µM of losartan shortened APD₉₀ by 13.03-14.56% (p<0.05 vs. Hypo-S).

Conclusion:

The above data indicate that the effect of losartan on the electrophysiological changes induced by stretch of atrial myocytes is associated with blocking of AT₁ receptor, and is beneficial for the treatment of AF that is often accompanied by the expansion of atrial myocytes and the increase of effective refractory period.

Irbesartan-mediated AT1 receptor blockade attenuates hyposmotic-induced enhancement of I Ks current and prevents shortening of action potential duration in atrial myocytes

INTRODUCTION

Stretch of the atrial membrane upregulates the slow component of delayed rectifier K(+) current (I(Ks)). Blockade of angiotensin II subtype 1 receptors (AT(1)R) attenuates this increase in I(Ks). The present study aimed to examine the effects of irbesartan, a selective AT(1)R blocker (ABR), on both the enhancement of I(Ks) and the shortening of action potential duration (APD) induced by stretching atrial myocytes for exploring the mechanisms underlying the prevention of atrial fibrillation (AF) by ABR.

METHODS

Hyposmotic solution (Hypo-S) was used to stretch guinea pig atrial myocytes. I(Ks) and APD were recorded using the whole-cell patch-clamp technique.

RESULTS

Irbesartan (1-50 μM) attenuated the Hypo-S-induced increase in I(Ks) and shortening of APD90. Hypo-S increased the I(Ks) by 113.4%, whereas Hypo-S + 1 μM irbesartan and Hypo-S + 50 μM irbesartan increased the I(Ks) by only 74.5% and 70.3%, respectively. In addition, Hypo-S shortened the APD(90) by 19.0%, whereas Hypo-S + 1 μM irbesartan and Hypo-S + 50 μM irbesartan shortened the APD90 by 12.1% and 12.0%, respectively.

CONCLUSION

The actions of irbesartan on electrical changes induced by stretching atrial myocytes are associated with blocking AT(1)R. These actions may be beneficial for treating AF.

KCNE1-KCNQ1 osmoregulation by interaction of phosphatidylinositol-4,5-bisphosphate with Mg2+ and polyamines

KCNQ1 osmosensitivity is of physiological and pathophysiological relevance in epithelial and cardiac cells, but the mechanism involved remains elusive. In COS-7 cells expressing the KCNE1-KCNQ1 fusion protein, extracellular hypoosmolarity and hyperosmolarity modify the channel biophysical parameters. These changes are consistent with hypoosmolarity increasing the level of membrane phosphatidylinositol-4,5-bisphosphate (PIP(2)), which in turn upregulates KCNE1-KCNQ1 channels. We showed that increasing PIP(2) levels with a water-soluble PIP(2) analogue prevented channel upregulation in hypoosmotic condition, suggesting a variation of the channel-PIP(2) interaction during channel osmoregulation. Furthermore, we showed that polyamines and Mg(2+), already known to tonically inhibit KCNQ channels by screening PIP(2) negative charges, are involved in the osmoregulatory process. Indeed, intracellular Mg(2+) removal and polyamines chelation inhibited the channel osmoregulation. Thus, the dilution of those cations during cell swelling might decrease channel inhibition and explain the channel upregulation by hypoosmolarity. To support this idea, we quantified the role of Mg(2+) in the osmodependent channel activity. Direct measurement of intracellular [Mg(2+)] variations during osmotic changes and characterization of the channel Mg(2+) sensitivity showed that Mg(2+) participates significantly to the osmoregulation. Using intracellular solutions that mimic the variation of Mg(2+) and polyamines, we were able to recapitulate the current amplitude variations in response to extracellular osmolarity changes. Altogether, these results support the idea of a modulation of the channel-PIP(2) interactions by Mg(2+) and polyamines during cell volume changes. It is likely that this mechanism applies to other channels that are sensitive to both osmolarity and PIP(2).