Selective elimination of IK,slow1 in mouse ventricular myocytes expressing a dominant negative Kv1.5α subunit

Although previous studies have revealed a role for the voltage-gated K+ channel α-subunit Kv1.5 ( KCNA5) in the generation of the 4-aminopyridine (4-AP)-sensitive component of delayed rectification in mouse ventricles ( IK,slow1), the phenotypic consequences of manipulating IK,slow1 expression in vivo in different (mouse) models are distinct. In these experiments, point mutations were introduced in the pore region of Kv1.5 to change the tryptophan (W) at position 461 to phenylalanine (F) to produce a nonconducting subunit, Kv1.5W461F, that is shown to function as a Kv1 subfamily-specific dominant negative (Kv1.5DN). With the use of the α-myosin heavy chain promoter to direct cardiac-specific expression, three lines of Kv1.5DN-expressing (C57BL6) transgenic mice were generated and characterized. Electrophysiological recordings from Kv1.5-DN-expressing left ventricular myocytes revealed that the micromolar 4-AP sensitive IK,slow1 is selectively eliminated. The attenuation of IK,slow1 is accompanied by increased ventricular action potential durations and marked QT prolongation. In contrast to previous findings in mice expressing a truncated (DN) Kv1.1 transgene; however, no electrical remodeling is evident in Kv1.5DN-expressing ventricular myocytes, and the (Kv1.5DN-induced) elimination of IK,slow1 does not result in spontaneous ventricular arrhythmias.

Delayed rectification in single cells isolated from guinea pig sinoatrial node

We have studied delayed rectifier K+ currents (IK) in cells isolated from the sinoatrial node (SAN) region of the guinea pig. Using whole cell patch-clamp procedures, we measured the voltage dependence of IK activation and IK kinetics and the IK equilibrium potential in 4.8 mM extracellular K concentration solutions. Experiments were designed to contrast properties of guinea pig SAN IK with those of IK recorded from SAN cells of the rabbit. We find that guinea pig SAN IK differs from IK recorded from single rabbit SAN cells in its activation threshold, and in the absence of inactivation of whole cell currents recorded over a wide voltage range. These results, along with the relative insensitivity of guinea pig SAN IK to E-4031 and lanthanum, suggest that under our experimental conditions, a strongly rectifying IK component (IK,r) is not the major component of delayed rectification in the guinea pig SAN, as it appears to be in SAN cells of the rabbit.

Studies of the cardiac-like action potential in crayfish giant axons induced by platinized tungsten metal electrodes

A lightly platinized tungsten (Pt-W) wire electrode, axially inserted into a crayfish giant axon, causes the development of cardiac-like action potentials with durations of up to 4 s. The plateau in membrane potential typically occurs within 10 min of the start of action potential elongation. The effect occurs without passing current through the Pt-W electrode and is temporally related to a dramatic decrease in intracellular pH (pHi). Such an effect cannot be induced by a decrease in pHi produced by equilibrating the axon with HCO3(−)-CO2 solution (pH6), and NH4Cl rebound or direct intracellular injection of PO4(3-) buffer (pH 4 X 5). Action potential elongation is accompanied by a block of delayed rectification and the possibility that inward rectification also develops cannot be ruled out. Plateau generation requires Na+ and Ca2+ inward currents as demonstrated by abolition of the plateau by [Na+]o or [Ca2+]o depletion or treatment with tetrodotoxin (TTX) or verapamil. The block of outward rectification by Pt-W requires external Na+ or Ca2+. Action potential elongation produced by 3,4-diaminopyridine is not sensitive to verapamil and the waveform is different from that produced by Pt-W. The data support the possibility that different classes of excitable membranes have similar channel populations and that the functional differences between them reside in the inhibitory or masking influences that are present in the microenvironments of the various membrane channels.

Presynaptic Depolarization Mediates Presynaptic Inhibition at a Synapse Between An Identified Mechanosensory Neurone and Giant Interneurone 3 in the First Instar Cockroach, Periplaneta Americana

Intracellular microelectrodes were used to study presynaptic inhibition at a cholinergic synapse between identified neurones: the lateral filiform hair sensory neurone (LFHSN) and giant interneurone 3 (GI3) in the terminal ganglion of the first instar cockroach Periplaneta americana. The LFHSN-GI3 synapse was shown to fulfil physiological criteria for monosynaptic transmission: the latency of the EPSPs was 1.4 ms and was constant during high-frequency firing of LFHSN; transmission was progressively and reversibly abolished by replacement of Ca2+ with Mg2+. Movement of the lateral filiform hair towards the cereal tip produced a burst of spikes in LFHSN and a burst of EPSPs in GI 3. Movement of the medial filiform hair towards the base of the cercus produced a burst of spikes in the medial filiform hair sensory neurone (MFHSN) and a burst of EPSPs in GI 2. EPSPs evoked in GI 3 by LFHSN spikes were inhibited during bursts of EPSPs in GI 2 which were evoked by MFHSN spikes. LFHSN was depolarized and its spikes were reduced in amplitude during spike bursts in MFHSN. Reduction in LFHSN spike amplitude reduced GI 3 EPSPs. This phenomenon was attributed, therefore, to presynaptic inhibition. The occurrence of presynaptic inhibition was dependent upon the degree of delayed rectification exhibited by the LFHSN axon. Hyperpolarization of LFHSN increased spike height, but did not increase the amplitude of GI 3 EPSPs. The delay between the onset of MFHSN-evoked EPSPs in GI 2 and MFHSNevoked depolarizations in LFHSN suggested that MFHSN does not synapse directly onto LFHSN. Neither depolarization nor hyperpolarization of GI 2 had any effect on MFHSN-mediated presynaptic inhibition of LFHSN-GI 3 transmission, therefore it was considered unlikely that GI 2 synapses onto LFHSN. Prolonged hyperpolarization lowered the LFHSN spike threshold and temporarily abolished presynaptic inhibition. Bursts of spikes in LFHSN mediated presynaptic inhibition of MFHSN-GI2 EPSPs. Mutual presynaptic inhibition by the FHSNs may have a functional significance in sharpening the boundaries of the GIs' directional sensitivities.