Subunit stoichiometry of a mammalian K+ channel determined by construction of multimeric cDNAs

The subunit stoichiometry of the mammalian K+ channel KV1.1 (RCK1) was examined by linking together the coding sequences of 2-5 K+ channel subunits in a single open reading frame and tagging the expression of individual subunits with a mutation (Y379K or Y379R) that altered the sensitivity of the channel to block by external tetraethylammonium ion. Two lines of evidence argue that these constructs lead to K+ channel expression only through the formation of functional tetramers. First, currents expressed by tetrameric constructs containing a single mutant subunit have a sensitivity to tetraethylammonium that is well fitted by a single site binding isotherm. Second, a mutant subunit (Y379K) that expresses only as part of a heteromultimer contributes to the expression of functional channels when coexpressed with a trimeric construct but not a tetrameric construct.

Evidence for cooperative interactions in potassium channel gating

Cloning and expression of voltage-activated potassium ion-channel complementary DNAs has confirmed that these channels are composed of four identical subunits, each containing a voltage sensor. It has been generally accepted that the voltage sensors must reach a permissive state through one or more conformational ('gating') transitions before the channel can open. To test whether each subunit gates independently, we have constructed cDNAs encoding four subunits on a single polypeptide chain, enabling us to specify the subunit stoichiometry. The gating of heterotetramers made up from combinations of subunits with different gating phenotypes strongly suggests that individual subunits gate cooperatively, rather than independently. Nonindependent subunit gating is consistent with measurements of the kinetics of K(+)-channel gating currents and in line with the widespread subunit cooperativity observed in other multisubunit proteins.

Flunarizine inhibits a high-threshold inactivating calcium channel (N-type) in isolated hippocampal neurons

The action of flunarizine on the high-threshold inactivating calcium channel (N-type) in hippocampal neurons of the rat was investigated using the whole-cell voltage clamp technique. Flunarizine reduced the currents at all test potentials, without shifting the peak of the current-voltage relationship along the voltage-axis. The drug did not affect the activation curve, but drastically decreased the slope conductance in the linear region of the current-voltage relationship. Block of the current by flunarizine occurred in a use-dependent way. Flunarizine was without effect when applied intracellulary, and the onset of action, when applied extracellularly, was slow (range of minutes). The Kd for the block by flunarizine obtained after 6 repetitive depolarizations at 0.2 Hz (pulse duration 150 ms) from -90 mV to 0 mV was 0.8 microM. In conclusion, we present electrophysiological evidence that flunarizine blocks the high-threshold inactivating Ca channel of hippocampal neurons of the rat. We discuss the possibility that flunarizine might inhibit neuronal transmitter release by means of this effect.

Agonistic and antagonistic effect of R56865 on the Na+ channel in cardiac cells

The effect of R56865, a benzothiazolamine derivative, on the Na+ channel was studied using different methods (indirect: Vmax measurements; direct: Na+ current under voltage clamp conditions and single channel analysis) on different cardiac preparations (rabbit Purkinje fibres, guinea-pig papillary muscles and guinea-pig ventricular single cells). Agonistic effects were obtained when hyperpolarized holding potentials and low frequency stimulation were used, antagonistic effects when depolarized holding potentials (inactivation block) and high frequency stimulation (use-dependent block) were applied. Recovery from block was voltage-dependent, and faster at hyperpolarized potentials. Burst duration was markedly shortened in DPI (DPI 201-106)-treated patches. The relation between the effects on the Na+ channel and the anti-digitalis effects are discussed.

Modulation of the T-type cardiac Ca channel by changes in proton concentration

Single-channel measurements and whole-cell experiments with the two suction electrode, voltage clamp technique were used to investigate the effects of external and internal proton concentrations on T-type Ca channels in heart muscle cells of the guinea pig. As in the L-type Ca channel, an increase in the external proton concentration decreases T-type currents, while external alkalinization enlarges the currents. In contrast to the L-type Ca channel, however, a change in the internal proton concentration does not modulate T-type Ca currents. The T-type Ca channel is much more sensitive to variations in pHo than the L-type Ca channel. By the combination of single-channel and whole-cell experiments we can conclude that the observed changes in macroscopic currents are due to (a) changes in the single-channel conductance and in the probability of the T-type Ca channel being open, and (b) the titration of the negative surface charges in the neighborhood of the T-type Ca channel with shifts of both the activation and inactivation processes of the channel. The pHo-induced changes in the maximal conductance (gmax) of the T-type Ca channel show an apparent pKa in the range of 7.1-7.5, while the titration of the negative surface charges near the channel shows an apparent pKa of 7.1 with a concomitant surface potential of -24.6 mV at 5.4 mM [Ca]o. These pKa values, less acid than the pKa values found for the pHo-induced, L-type Ca channel modulation, might imply a physiological importance of this novel type of channel modulation.

A combined study of sodium current and T-type calcium current in isolated cardiac cells

Sodium currents (INa) and T-type calcium currents (ICa,T) of isolated guinea-pig ventricular myocytes were recorded using the whole-cell voltage-clamp technique. Separation of the two currents was obtained by using the difference current method in the presence and absence of 2 mM extracellular Na (Nao). Time to peak and the time constant of inactivation of. INa were about 5 times faster than that of ICa,T (test potential -30 mV), and ICa,T had an activation range positive to -50 mV, were inactivated at -50 mV, and their current/voltage relationships peaked at -22.3 +/- 1.8 mV (n = 18) and -29.3 +/- 0.5 mV (n = 18) respectively, with a reversal potential of +40.3 +/- 4 mV (n = 18) and +30 +/- 10 mV (n = 18), respectively [2 mM Nao; 5.4 mM extracellular Ca (Cao)]. INa was blocked by 30 microM tetrodotoxin (TTX), 500 microM lidocaine, partly inhibited by 1 mM amiloride, but not affected by 100 microM nickel (Ni). ICa,T was neither affected by 30 microM TTX nor 500 microM lidocaine, but blocked by 100 microM Ni, 1 mM amiloride, 10 microM R 56865 and use-dependently reduced by 5 microM flunarizine. Adenosine (500 microM) affected neither INa nor ICa,T, whereas 1 microM isoprenaline did not affect ICa,T, but slightly increased INa. Our results demonstrate that the characteristics of ICa,T are not affected by the concomitant activation of INa, and vice versa. We conclude that ICa,T are not Ca currents through Na channels.

Mechanism of cardiac T-type Ca channel blockade by amiloride

Whole cell clamp experiments were used to study the mechanism of action of amiloride on cardiac T-type Ca channels in guinea pig ventricular myocytes. Two hundred fifty microM amiloride blocked the T-type Ca channel for approximately 50%, whereas this concentration did not suppress the L-type Ca channel. The Kd value for the block of the T-type Ca channel was 233 microM and the Hill coefficient was 1.1. The drug showed a rapid onset of action and a quick wash out with complete reversibility. Amiloride blocked the T-type Ca channel without frequency-dependency, i.e., block was established at the holding potential (-90 mV) without being dependent on activation and inactivation of the channel. Amiloride reduced the amplitude of the T-type Ca current at all potentials without changing steady-state activation and inactivation. Amiloride did not affect the T-type Ca current when the drug was applied intracellularly via the pipette solution. In summary, amiloride blocked the T-type Ca channel in a channel state- and voltage-independent way.

Modulation of cardiac Na channels by angiotensin II

The modulation of Na channels by the vasoactive peptide angiotensin II (AT II) has been studied in isolated ventricular cells of guinea pigs using the patch clamp technique. In cell-attached patches the maximal probability of the channel being open was increased in a concentration range between 0.05 and 1 microM, but decreased at higher concentrations. A maximal increased of 2.5 +/- 0.86 was found at 1 microM AT II. The increase in the probability of the channel being open was due to a decrease in the number of nulls. In all affected cells (n = 17) we observed a delayed inactivation after application of AT II at concentrations between 0.05 and 10 microM. At -30 mV, the time constant of inactivation increased from 1.1 +/- 0.1 ms (controls) to 5.6 +/- 1.6 ms (10 microM AT II). This effect was due to an increased number of openings per sweeps. No significant effect on the mean open time and the first latency were observed. However, due to pronounced bursting, the averaged closed time was significantly increased from 0.8 +/- 0.1 ms to 1.3 +/- 0.1 ms in the presence of 1 microM AT II at -30 mV. An effect of AT II on cardiac Na channels via protein kinase C is discussed.

The T-type Ca channel in guinea-pig ventricular myocytes is insensitive to isoproterenol

Whole cell clamp experiments and single channel measurements were used to study whether the T-type Ca channel in guinea-pig ventricular myocytes is sensitive to beta-adrenergic stimulation. In whole cell clamp experiments we determined the T-type Ca current by measuring the difference between the currents at a test potential of -30 mV in Na-free solution, starting from a holding potential either at -90 or at -50 mV. Application of 1 microM isoproterenol (ISO) did not change the difference current, while the L-type Ca current was enhanced during application of the drug. In cell attached patch experiments with 110 mM CaCl2 in the pipette solution, single channel currents were observed which were due to opening of the T-type Ca channel. Application of 4 microM ISO to the bath solution did neither change the appearance of the channel openings, nor the ensemble averaged currents. It is concluded that the T-type Ca channel in guinea-pig ventricular cells is insensitive to ISO.

Differential effects of verapamil and flunarizine on cardiac L-type and T-type Ca channels

Whole cell experiments were used to study whether the L-type and the T-type Ca channel in guinea-pig ventricular myocytes are blocked similarly by verapamil and flunarizine. The L-type current is blocked by 5 mumol/l verapamil and 5 mumol/flunarizine in a use-dependent way, and block can be relieved by hyperpolarizing pulses in a potential-dependent way. The T-type current is not affected by 10 mumol/l verapamil while it is blocked by 10 mumol/l flunarizine in a use-dependent way. Verapamil selectively blocks the L-type channel in contrast to flunarizine.