Biphasic effects of cell volume on excitation-contraction coupling in rabbit ventricular myocytes

Abstract

.-We studied the effects of osmotic swelling on the components of excitation-contraction coupling in ventricular myocytes. Myocyte volume rapidly increased 30% in hyposmotic (0.6T) solution and was constant thereafter. Cell shortening transiently increased 31% after 4 min in 0.6T but then decreased to 68% of control after 20 min. In parallel, the L-type Ca 2ϩ current (ICa-L) transiently increased 10% and then declined to 70% of control. Similar biphasic effects on shortening were observed under current clamp. In contrast, action potential duration was unchanged at 4 min but decreased to 72% of control after 20 min. Ca 2ϩ transients were measured with fura 2-AM. The emission ratio with excitation at 340 and 380 nm (f 340/f380) decreased by 12% after 3 min in 0.6T, whereas shortening and I Ca-L increased at the same time. After 8 min, shortening, I Ca-L, and the f340/f380 ratio decreased 28, 25, and 59%, respectively. The results suggest that osmotic swelling causes biphasic changes in I Ca-L that contribute to its biphasic effects on contraction. In addition, swelling initially appears to reduce the Ca 2ϩ transient initiated by a given ICa-L, and later, both I Ca-L and the Ca 2ϩ transient are inhibited.osmolarity; action potential duration; calcium current; calcium transient; cell shortening SWELLING OF CARDIAC MYOCYTES is a prominent aspect of the response to ischemia-reperfusion and elective cardioplegia and also may arise in renal insufficiency and syndromes with inappropriate secretion of antidiuretic hormone. Altered myocyte hydration has important functional consequences. Osmotic perturbations modulate both electrical activity (for reviews, see 34,38,42) and the ability of cardiac muscle to contract (5,16,17). Although facets of the contractile response to osmotic swelling are known from studies on multicellular preparations, little mechanistic information is available at the single cell level to explain the implications of myocyte swelling for excitation-contraction (E-C) coupling. Moreover, the effect of swelling on L-type Ca 2ϩ current (I Ca-L ), a critical aspect of E-C coupling, remains controversial. Under ruptured patch voltage-clamp conditions, for example, swelling is reported to enhance I Ca-L in rabbit atrial and sinoatrial node cells (22), depress I Ca-L in rat ventricular cells (4), and to have no significant effect in guinea pig (13, 31) and canine (44) ventricular cells. A swelling-induced enhancement of I Ca-L also has been claimed based on fura 2 measurements of diltiazem-sensitive Ca 2ϩ influx (36). It is unclear whether these differences in the response of I Ca-L to myocyte swelling arise from species differences or from methodological issues, such as the composition of the patch pipette solution, the effectiveness of cell dialysis, or the variable extent of myocyte swelling under ruptured-patch conditions (6, 33). Several other species differences in E-C coupling, including the sensitivity of contraction to inhibition of the sarcoplasmic reticulum (SR), are well known (2).The present stu...

Effect of osmotic stress on spontaneous calcium sparks in rat ventricular myocytes

“…However, when the extracellular solution was further changed to 0.6 T, [Ca 2+ ] i was returned to the baseline (Figure 2Ba). Similarly, the increase of [Ca 2+ ] i induced by 1 T was absent in Ca 2+ free extracellular solution (Figure 2Bb), indicating that it might be attributed to enhanced Ca 2+ entry or reduced Ca 2+ extrusion by Na + -Ca 2+ exchanger [1,7,16] . All of these effects of osmotic stress on [Ca 2+ ] i were reversible.…”

“…Furthermore, it has been shown that the increased blood tonicity in streptozotocin-induced diabetic rats leads to shrinkage of ventricular myocytes [5] . Many previous studies have indicated that both electrical and contractile properties of ventricular myocytes are affected by osmotic stress [1,[5][6][7][8][9] . For example, it has been consistently found that L-type Ca 2+ currents, Ca 2+ transients and contraction evoked by action potential are decreased by prolonged exposure to hyposmotic medium [1,7] .…”

“…Many previous studies have indicated that both electrical and contractile properties of ventricular myocytes are affected by osmotic stress [1,[5][6][7][8][9] . For example, it has been consistently found that L-type Ca 2+ currents, Ca 2+ transients and contraction evoked by action potential are decreased by prolonged exposure to hyposmotic medium [1,7] . It is well established that Ca 2+ sparks are elementary events of Ca 2+ release mediated by ryanodine receptors/Ca 2+ release channels (RyRs) on the sarcoplasmic reticulum (SR) of muscle cells [10] .…”

Hyposmotic challenge modulates function of L-type calcium channel in rat ventricular myocytes through protein kinase C

“…Brette et al had reported that in rat ventricular myocytes osmotic swelling decreases I Ca,L [16] . On the other hand, Li et al had found that in rabbit ventricular myocytes hyposmotic swelling induces biphasic changes, an increase followed by decrease, of I Ca,L under perforated whole-cell patch-clamp conditions [18] . In the present study, we found that osmotic swelling also induced biphasic changes of I Ca,L , a transient increase followed by sustained decrease, in rat ventricular myocytes (Figure 1).…”

“…For example, some studies have shown clear increases in I Ca,L [12][13][14] , while others have shown a decrease in I Ca,L [15,16] or no effect [10,17] . Some authors have even demonstrated biphasic effects on I Ca,L , an increase followed by a more sustained decrease [18] . Previous reports indicated that osmotic swelling increases I Ca,L in neonatal rat ventricular myocytes [14] , but decreases I Ca,L in adult rat ventricular myocytes [16] .…”