Two types of calcium channels in guinea pig ventricular myocytes.

Abstract

In cardiac muscle, Ca2+ plays a key role in regulation of numerous processes, including generation of the action potential and development of tension. The entry of Ca21 into the cell is regulated primarily by voltage-gated channels in the membrane. Until recently, it was felt that only one type of Ca2+ channel existed in cardiac ventricular muscle. Experiments reported here suggest that in isolated guinea pig ventricular myocytes, there are two distinct types of Ca2+ channels with markedly different activation thresholds, inactivation kinetics, and sensitivities to inorganic and organic Ca2+ channel blockers. The channels were also distinguished based on their response to increased frequency of clamping such that the current through the low-threshold channel decreased while that through the high-threshold channel increased. In a few cells, the current through both channels was enhanced by isoproterenol, a J3-adrenergic agonist, but only the high-threshold channel was enhanced by the Ca2+-channel agonist Bay K 8644. Thus, isolated guinea pig ventricular myocytes appear to have two types of Ca2+ channels distinguished by various criteria.In amphibian and mammalian cardiac muscle, a slow inward current (Iji) capable of transporting either Ca2l or Na+ has been identified (1-4). This current, which activates near -30 mV and inactivates slowly with two time constants (5, 6), appears to contribute to the maintenance of the plateau of the cardiac action potential and to the generation of tension. Recently, evidence for two distinct populations of Caa2+ channels has been put forward in a wide variety ofnoncardiac cells, including starfish eggs (7), pituitary cells (8), dorsal root ganglion neurons (9, 10), fresh water ciliates (11), and polychaete eggs (12). The biological grounds for the development of two types of calcium channels in the same neuronal cell, however, are not as yet clear. In cardiac muscle, the presence of two types of calcium channels may play differential roles in the generation of pacemaking activity, maintenance of the plateau, or release of calcium from intracellular pools. Recently, data have been presented that show the presence of two types of calcium channels in canine atrial myocytes (13). Single-channel data from isolated guinea pig ventricular myocytes also suggest the existence of two calcium channel types (14). In this communication, we provide further evidence for the presence of two types of calcium channels in single internally dialyzed guinea pig ventricular myocytes using only calcium (5-15 mM) as the charge carrier. Our data clearly show that the two calcium channel types can be distinguished based on their activation threshold, inactivation kinetics, response to clamping frequency, and pharmacological sensitivity to various inorganic and organic calcium-channel blockers. METHODSGuinea pig ventricular myocytes were isolated by using a newly developed collagenase-protease technique (15). Guinea pigs (body weight, 200-400 g) were heparinized (10 units/g) 10 min prior to anesthe...

Potent vasodilatory with minor cardiodepressant actions of mibefradil in human cardiac tissue

“…-channels were found, Tand L-type Ca 2-channels (Bean, 1985;Nilius et al, 1985;Mitra & Morad, 1986;Beuckelmann, 1997 -channel antagonists of the 1,4-dihydropyridine type, for example nifedipine (Bean et al, 1986). In high concentrations (3 mmol l 71 ) 1,4 dihydropyridines can inhibit the T-type Ca 2+ -channels as well (Bean, 1985;Nilius et al, 1985;Mitra & Morad, 1986;Hagiwara et al, 1988).…”

“…In high concentrations (3 mmol l 71 ) 1,4 dihydropyridines can inhibit the T-type Ca 2+ -channels as well (Bean, 1985;Nilius et al, 1985;Mitra & Morad, 1986;Hagiwara et al, 1988). Selective inhibitory responses for the T-type Ca 2+ -channel have been shown for the insecticide tetramethrin (Hagiwara et al, 1988).…”

Differences in Apparent Pore Sizes of Low and High Voltage-activated Ca2+ Channels

“…In contrast to the better studied high voltageactivated (HVA) 1 Ca 2ϩ channels (L-, N-, P/Q-, and R-type), T-type channels activate at relatively negative membrane potentials and are often referred to as "low voltage-activated" (LVA). The distinctive permeation properties of T-type Ca 2ϩ channels were critical for their original identification as distinct entities (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). In native preparations, T-type channels are equally permeable to Ca 2ϩ and Ba 2ϩ ions, unlike all known HVA channels, for which unitary Ba 2ϩ fluxes are larger than those supported by Ca 2ϩ (17).…”

Voltage‐dependence of ryanodine‐sensitive component of contraction in A23187‐ and isoprenaline‐treated cardiac muscles

“…This channel has been defined as a T-type of Ca channel (Hume & Giles, 1983;Bean, 1985;Nilius et al, 1985;Mitra & Morad, 1986). From these studies, a T-type of Ca channel is characterized by the low-activation threshold, the rapid inactivation, and the inactivation at low holding potential (almost abolished at a holding potential of -50 mV).…”

“…From the recent studies on inward current. On the other hand, Fabiato (1985a), and Morad & Cleemann (1987) have proposed that Ca release may be triggered by Ca influx through fast Ca channels (corresponding to T-type of Ca channels), different from slow channels which have been described by Hume & Giles (1983), Bean (1985), Nilius et al (1985), and Mitra & Morad (1986). Thus, although Ca-induced Ca release may be a prevailing theory of cardiac excitationcontraction coupling, the exact role of Ca inward currents on the Ca-induced Ca release process in intact cardiac muscle is the subject of some controversy. Recently, the Ca ionophore, A23187, has been reported to produce two components of contraction each component of which could be separately observed by use of specific pharmacological interventions (Kondo, 1986).…”