Voltage-gated Ca2+ currents in the human pathophysiologic heart: a review.

The L-type Ca2+ current (I(Ca-L)) plays a key role in the cardiac excitation-contraction (E-C) coupling. Thus, it is a major target for many transmitters and hormones modulating cardiac function and, therefore, for pharmacological drugs to regulate inotropy. Ca2+ (and other) ion currents are commonly studied in animal tissues for practical reasons. Investigations in human cardiomyocytes started extensively only ten years ago with the development of patch-clamp techniques, enzymatic cell dissociation procedures, and surgical techniques. These studies have already provided valuable information concerning the nature, biophysics, pharmacology and regulation of human cardiac ionic currents in normal and diseased tissues. Interesting advances have been made to understand the role of I(Ca-L) in the development of chronic atrial fibrillation (AF). Alterations of single channel activity and regulation of macroscopic I(Ca-L) have also been found in heart failure (HF), ugh some of the data are divergent and puzzling. The T-type Ca2+ current (I(Ca-T)) has never been recorded in human cardiomyocytes. After a rapid overview of the basic properties of human cardiac Ca2+ currents, we focus on selected aspects of pathophysiology that are still unsolved.

Microtubule disruption by colchicine reversibly enhances calcium signaling in intact rat cardiac myocytes.

Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P<0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P<0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.

Microtubule disruption modulates Ca(2+) signaling in rat cardiac myocytes.

Microtubules have been shown to alter contraction in cardiac myocytes through changes in cellular stiffness. However, an effect on excitation-contraction coupling has not been examined. Here we analyze the effects of microtubule disruption by 1 micromol/L colchicine on calcium currents (I(Ca)) and [Ca(2+)](i) transients in rat ventricular myocytes. I(Ca) was studied using the whole-cell patch-clamp technique. Colchicine treatment increased I(Ca) density (peak values, -4.6+/-0.4 and -9.1+/-1.3 pA/pF in 11 control and 12 colchicine-treated myocytes, respectively; P<0.05). I(Ca) inactivation was well fitted by a biexponential function. The slow component of inactivation was unchanged, whereas the fast component was accelerated after colchicine treatment (at -10 mV, 11.8+/-1.0 versus 6.7+/-1.0 ms in control versus colchicine-treated cells; P<0.005). [Ca(2+)](i) transients were analyzed by fluo-3 epifluorescence simultaneously with I(Ca). Peak [Ca(2+)](i) transients were significantly increased in cardiac myocytes treated with colchicine. The values of F/F(0) at 0 mV were 1.1+/-0.02 in 9 control cells and 1.4+/-0.1 in 11 colchicine-treated cells (P<0.05). beta-Adrenergic stimulation with 1 micromol/L isoproterenol increased both I(Ca) and [Ca(2+)](i) transient in control cells. However, no significant change was induced by isoproterenol on colchicine-treated cells. Colchicine and isoproterenol effects were similar and not additive. Inhibition of adenylyl cyclase by 200 micromol/L 2'-deoxyadenosine 3'-monophosphate blunted the colchicine effect. We suggest that beta-adrenergic stimulation and microtubule disruption share a common pathway to enhance I(Ca) and [Ca(2+)](i) transient.