The Involvement of Calcium Mobilization in the Calcium-Activated Potassium Currents Activated by Hyposmotic Swelling in Gastric Antral Circular Myocytes of the Guinea-Pig.

Abstract

The cells of vertebrates exposed to hyposmotic media, initially swell by osmotic water equilibration, but subsequently regulate their volume (regulatory volume decrease, RVD) by a loss of KCl and water (for review, see references [1][2][3]). The mechanisms of osmoregulatory K ϩ and Cl Ϫ efflux have not been completely understood. However, K ϩ and Cl Ϫ channels have been proposed to play an important role in RVD in various kinds of cells, such as those in frog urinary bladder [4], human lymphocytic cells [5], human epithelial cells [6], human platelets [7], and epithelial cells of frog skin [8].Both extracellular and intracellular [Ca 2ϩ ] are too low in animals to be useful as osmolytes. However, the Ca 2ϩ role during RVD has been recognized for many years. Many patch-clamp studies have found that the activation of volume-regulatory K ϩ channels is controlled by Ca 2ϩ (for review, see references [9,10]). Some experiments indicated that I K(Ca) was activated by an influx of Ca 2ϩ when the cell was superfused with hyposmotic solution [6,11,12]. Other experiments have shown that I K(Ca) is activated by an intracellular Ca 2ϩ signal that is released from intracellular calcium stores during RVD [13]. But the effect of hyposmotic swelling on I K(Ca) and the role of intracellular or extracellular Ca 2ϩ in gastric smooth muscle cells has not been investigated.In our previous study, we reported that the voltageoperated calcium current (I Ca ) was increased [14] and that the volume-sensitive chloride current (I Cl ) was activated by hyposmotic swelling [15] in gastric antral myocytes of the guinea-pig. In the present study, we Key words: gastric smooth muscle cell, Ca 2ϩ -activated K ϩ current, Ca 2ϩ -induced Ca 2ϩ release, swelling.Abstract: In our study of the effects of hyposmotic swelling on the Ca 2ϩ -activated potassium currents [I K(Ca) ] and its mechanism, we employed the whole-cell patch clamp technique using the gastric antral circular myocytes of the guinea-pig. Hyposmotic swelling efficiently increased I K(Ca) , and the extent of changes in I K(Ca) was sharply dependent on the osmolarity of the perfusion solutions. When the calcium-free solution (EGTA 10 M added in calcium-free solution) was superfused, I K(Ca) was not increased by the hyposmotic swelling. Gadolinium (Gd 3ϩ ) 100 nM, a blocker of the stretch-activated nonselective cation channel, blocked the activation of I K(Ca) induced by hyposmotic swelling, but nicardipine 5 M (the Ltype calcium channel blocker) did not. Heparin 3 mg/ml, a potent inhibitor of inositol triphosphate receptor (InsP 3 R), did not inhibit the response, and caffeine 1 mM (the agonist for calcium-induced calcium release [CICR]) imitated the effect of hyposmotic swelling. Ryanodine (15 M), markedly inhibited the effect. These results suggest that hyposmotic swelling activates I K(Ca) , and the activation is associated with CICR, which is triggered by extracellular calcium influx through the stretch-activated channel (SA channel).

Role of arachidonic acid in hyposmotic membrane stretch-induced increase in calcium-activated potassium currents in gastric myocytes1

“…However, Murthy et al [20] found that AA induces Ca 2+ influx, which triggers CICR in longitudinal smooth muscle of the intestine. Our previous study also demonstrated that hyposmotic membrane stretch activates I K(Ca) via CICR in gastric myocytes [15] . In the present study, the roles of AA and its metabolites in the relationships among hyposmotic membrane stretch-induced increase in I K(Ca) , extracellular Ca 2+ and intracellular calcium mobilization were investigated.…”

“…The AA-induced increase in I K(Ca) was completely blocked by the removal of extracellular calcium, and the changes in the percentage of I K(Ca) were 146.30%±10.4% and 95.64%±11.7% in the presence or absence of extracellular calcium, respectively (n=6, Figure 6A). Our previous study indicated that hyposmotic membrane stretch activated L-type calcium currents [12] and calcium-activated potassium currents via extracellular calcium influx through SAC in gastric myocytes of guinea pig [15,16] . We therefore examined the relationship between the AA-induced increase in I K(Ca) and L-type calcium channels or SAC.…”

“…Our previous study demonstrated that I K(Ca) was activated by hyposmotic membrane stretch and Ca 2+ signaling played an important role in the process in gastric antral circular myocytes of guinea pig [15,16] . Under hyposmotic conditions, extracellular calcium influx through SAC triggered CICR, and intracellular free calcium then activated I K(Ca) .…”

“…In our previous study, AA and other unsaturated fatty acids directly inhibited calcium currents [12] and muscarinic currents [13] . For both AA and hyposmotic membrane stretch-activated I K(Ca) [14,15] , activation by hyposmotic membrane stretch is associated with calcium-induced calcium release (CICR) [16] , which is triggered by extracellular calcium influx through the stretch-activated channels (SAC) in gastric antral circular myocytes of the guinea pig. However, the roles of AA and other unsaturated fatty acids in the process of I K(Ca) activation by membrane stretch in gastric myocytes remains unclear.…”

Effects of Dendroaspis Natriuretic Peptide on Calcium-Activated Potassium Current and Its Mechanism

“…In gastric antral circular myocytes of guinea pigs, depolarization pulses activated two types of K + currents, I K(Ca) and I K(V) [22]. To eliminate I K(V) , we used three approaches in this study: (1) the pipette solution contained egtazic acid (0.1 mM); (2) the bath contained 4-AP 10 mM; and (3) STOCs were significantly blocked by TEA and CHTX (Fig.…”