Biomedical subjects

T Leinders

Publications and source records attributed to T Leinders.

6 recordsLinked to original sources

PubMed1992

Single Ca(2+)-activated K+ channels in human erythrocytes: Ca2+ dependence of opening frequency but not of open lifetimes.

Using the patch-clamp technique single-channel properties of Ca(2+)-activated K+ (CaK) channels were investigated in inside-out membrane patches of human erythrocytes. In a physiological K+ gradient (5 mM K+ externally: 150 mM K+ internally) the single CaK channel conductance is 15 pS in the membrane potential range of -40 to +40 mV. The channel open probability, opening frequency and open and closed time distributions are voltage-independent. The open probability and the opening frequency of the CaK channel depend on [Ca2+]i and increase between 0.5 and 60 microM Ca2+ from approx. 10% to 90% of the maximum value obtained at 115 microM. The relation between open probability and [Ca2+]i can be described by a sigmoid concentration-effect curve with an EC50 of 4.7 microM and a slope factor of 1. Independent of [Ca2+]i open time distributions yield two time constants of 5.3 and 22 ms. The relative amplitudes of the fast and slow components of the open time histogram as well as the maximum open probability and the maximum opening frequency of CaK channels vary considerably. In addition, CaK channels in multiple channel patches are highly interdependent. It is concluded that the Ca(2+)-dependence of CaK channels in human erythrocytes is due to the modulation of opening frequency by internal Ca2+. The results are consistent with a classical receptor-agonist model in which ligand interaction kinetics are much faster than channel gating.

Calcium↗

PubMed1992

Distinct metal ion binding sites on Ca(2+)-activated K+ channels in inside-out patches of human erythrocytes.

Effects of Cd2+, Co2+, Pb2+, Fe2+ and Mg2+ (1-100 microM) on single-channel properties of the intermediate conductance Ca(2+)-activated K+ (CaK) channels were investigated in inside-out patches of human erythrocytes in a physiological K+ gradient. Cd2+, Co2+ and Pb2+, but not Fe2+ and Mg2+, were able to induce CaK channel openings. The potency of the metals to open CaK channels in human erythrocytes follows the sequence Pb2+, Cd2+ > Ca2+ > or = Co2+ >> Mg2+, Fe2+. At higher concentrations Pb2+, Cd2+ and Co2+ block the CaK channel by reducing the opening frequency and the single-channel current amplitude. The potency of the metals to reduce CaK channel opening frequency follows the sequence Pb2+ > Cd2+, Co2+ >> Ca2+, which differs from the potency sequence Cd2+ > Pb2+, Co2+ >> Ca2+ to reduce the unitary single-channel current amplitude. Fe2+ reduced the channel opening frequency and enhanced the two open times of CaK channels activated by Ca2+, whereas up to 100 microM Mg2+ had no effect on any of the measured single-channel parameters. It is concluded that the activation of CaK channels of human erythrocytes by various metal ions occurs through an interaction with the same regulatory site at which Ca2+ activates these channels. The different potency orders for the activating and blocking effects suggest the presence of at least one activation and two blocking sites. A modulatory binding site for Fe2+ exists as well. In addition, the CaK channels in human erythrocytes are distinct from other subtypes of Ca(2+)-activated K+ channels in their sensitivity to the metal ions.

Binding Sites↗

PubMed1992

Divalent cations activate small- (SK) and large-conductance (BK) channels in mouse neuroblastoma cells: selective activation of SK channels by cadmium.

Effects of Cd2+, Co2+, Fe2+ and Mg2+ (1 microM and 100 microM) and Pb2+ (1 microM and 90 microM) on single-channel properties of the small-conductance (SK) and large-conductance (BK) Ca(2+)-activated K+ channels were investigated in inside-out patches of N1E-115 mouse neuroblastoma cells. Cd2+, Co2+ and Pb2+, but not Fe2+ and Mg2+, cause SK channel opening. The potency of the metals in enhancing the SK channel-open probability follows the sequence Cd2+ approximately Pb2+ > Ca2+ > Co2+ >> Mg2+, Fe2+. The four metals that cause SK channel opening are equipotent in enhancing the opening frequency of SK channels. The BK channel is activated by Pb2+ and Co2+, whereas Cd2+, Fe2+ and Mg2+ are ineffective. The potency of the metals in enhancing BK channel-open probability, open time and opening frequency follows the sequence Pb2+ > Ca2+ > Co2+ >> Cd2+, Mg2+, Fe2+. The results show that SK channels are much more sensitive to Cd2+ than BK channels and indicate that Cd2+ is a selective agonist of SK channels. It is concluded that the various metal ions bind to the same regulatory site(s) at which Ca2+ activates the SK and BK channels under physiological conditions. The different potency sequences of metal ions with respect to BK and SK channel activation indicate that the regulatory sites of these Ca(2+)-activated K+ channels have distinct chemical and physical properties.

Animals↗

PubMed1992

Ca2+ dependence of small Ca(2+)-activated K+ channels in cultured N1E-115 mouse neuroblastoma cells.

Single-channel properties of Ca(2+)-activated K+ channels have been investigated in excised membrane patches of N1E-115 mouse neuroblastoma cells under asymmetric K+ concentrations at 0 mV. The SK channels are blocked by 3 nM external apamin, are unaffected by 20 mM external tetraethylammonium (TEA) and have a single-channel conductance of 5.4 pS. The half-maximum open probability and opening frequency of SK channels are observed at 1 microM internal Ca2+. Concentration/effect curves of these parameters are very steep with exponential slope factors between 7 and 13. Open-time distributions demonstrate the existence of at least two open states. The mean short open time increases with [Ca2+]i, whereas the mean long open time is independent of [Ca2+]i. At low [Ca2+]i the short-lived open state predominates. At saturating [Ca2+]i the number of long-lived openings is more enhanced than the number of short-lived openings and both open states occur equally frequently. The opening frequency as well as the open times of SK channels are independent of the membrane potential in the range of -16 to +40 mV. The results indicate that activation of K+ current through SK channels is mainly determined by the Ca(2+)-dependent single-channel opening frequency. BK channels in N1E-115 cells are insensitive to 100 nM external apamin, are sensitive to external TEA in the millimolar range and have a single-channel conductance of 98 pS. Half-maximum open probability and opening frequency of the BK channel are observed at 7.5-21 microM internal Ca2+. The slope factors of concentration/effect curves range between 1.7 and 2.9. As the BK channel open time is markedly enhanced at raised [Ca2+]i, the Ca2+ dependence of the current through BK channels is determined by the single-channel opening frequency as well as the open time. SK as well as BK channels appear to be clustered and interact in a negative cooperative manner in multiple channel patches. The differences in Ca2+ dependence suggest that BK channels are activated by a local high [Ca2+]i associated with Ca2+ influx, whereas SK channels may be activated by Ca2+ released from internal stores as well.

Animals↗

PubMed1989

The action of pyrethroids on sodium channels in myelinated nerve fibres and spinal ganglion cells of the frog.

The interaction of pyrethroids with the voltage-dependent sodium channel was studied in voltage-clamped nodes of Ranvier and isolated spinal ganglion neurons of the clawed frog, Xenopus laevis. In the node, pyrethroids prolonged the sodium tail current associated with a step repolarization of the membrane. It was found that the amplitude of the slow, pyrethroid-induced, sodium tail current (PIT) first increased and then decreased as a function of the duration of membrane depolarization (to -5 mV). This decrease of the PIT amplitude was absent when depolarizations to the sodium equilibrium potential (+40 mV) were used. Measurements of changes in sodium reversal potential indicated that sodium ion depletion in the perinodal space is largely responsible for the inactivation of the pyrethroid-modified sodium current. Inactivation is not completely abolished by pyrethroid treatment since the probability of channel opening, measured in membrane patches excised from spinal ganglion cells, decreased slowly during prolonged depolarization. Analysis of unitary currents indicated that both activation and inactivation are retarded by pyrethroids. The arrival of sodium channels in the pyrethroid-modified open state followed a time course that was slower than both activation and inactivation of unmodified sodium channels. Our findings indicate that sodium channels are modified when in the closed resting state and that both opening and closing kinetics are delayed by pyrethroids.

Action Potentials↗

PubMed1989

Ovariectomy and subchronic estradiol-17 beta administration decrease dopamine D1 and D2 receptors in rat striatum.

Ovariectomy and subchronic estradiol-17 beta cause a down-regulation of dopamine D1 and, to a lesser extent, D2 receptors in rat striatum. An intracellular mechanism mediates the DA receptor down-regulation, as various estrogens do not interact with membrane-bound DA receptors in vitro. A common denominator, e.g. enhanced DA turnover, is suggested to mediate the estradiol-induced DA receptor down-regulation. Ovarian factors other than estradiol are additionally proposed to be involved in the regulation of striatal DA receptors.

Animals↗