Biomedical subjects

C J Kros

Publications and source records attributed to C J Kros.

23 records · Page 2Linked to original sources

PubMed1994

Block by amiloride and its derivatives of mechano-electrical transduction in outer hair cells of mouse cochlear cultures.

1. The effects of amiloride and amiloride derivatives on mechano-electrical transducer currents in outer hair cells of the cultured neonatal mouse cochlea were examined under whole-cell voltage clamp. 2. At -84 mV transducer currents were reversibly blocked by the extracellular application of the pyrazinecarboxamides amiloride, benzamil, dimethylamiloride, hexamethyleneiminoamiloride, phenamil and methoxynitroiodobenzamil with half-blocking concentrations of 53, 5.5, 40, 4.3, 12 and 1.8 microM, respectively. Hill coefficients were determined for all but the last of these compounds and were 1.7, 1.6, 1.0, 2.2 and 1.6, respectively, suggesting that two drug molecules co-operatively block the transducer channel. 3. Both the structure-activity sequence for amiloride and its derivatives and the mechanism of the block of the transducer channel appear to be different from those reported for the high-affinity amiloride-sensitive epithelial Na+ channels but similar to those of stretch-activated channels in Xenopus oocytes. 4. The block by all pyrazinecarboxamides was voltage dependent with positive membrane potentials releasing the block. The form of the voltage dependence is consistent with a voltage-independent binding of the drug to a site that is accessible at hyperpolarized but not at depolarized potentials, suggesting that the transducer channel undergoes a voltage-dependent conformational change. The channel was not blocked by 1 mM amiloride from the intracellular side at either negative or positive membrane potentials. 5. The kinetics of the block were studied using force steps or voltage jumps. The results suggest that the drug binding site is only accessible when the transducer channel is open (open-channel block) and that the channel cannot close when the drug molecules are bound. 6. The time dependence and voltage dependence of the block together reveal that the transducer channel has at least two open conformational states, the transition between which is voltage dependent.

Amiloride↗

PubMed1993

Potassium currents in type II vestibular hair cells isolated from the guinea-pig's crista ampullaris.

Type II vestibular hair cells were isolated from cristae ampullares of guinea-pig and maintained in vitro for 2-3 h. Outward membrane currents were studied under whole-cell voltage-clamp conditions. Type II hair cells had resting potentials of about -45 mV. Depolarizing voltage steps from a holding potential of -80 or -90 mV induced time- and voltage-dependent outward currents which slowly decayed to a sustained level. Tail currents reversed at about -70 mV, indicating that the outward currents were mainly carried by potassium ions. The currents had an activation threshold around -50 mV. The transient component was completely removed by a depolarizing pre-pulse positive to -10 mV. While bath application of 4-aminopyridine (5 mM) reduced both components, extracellular tetraethylammonium (10 mM) or zero calcium preferentially diminished the sustained current. We conclude that at least two potassium conductances are present, a delayed rectifier with a relatively fast inactivation and a calcium-dependent potassium current. Depolarizing current injections induced an electrical resonance in the voltage responses, with a frequency of 25-100 Hz, larger currents causing higher frequencies.

4-Aminopyridine↗

PubMed1992

[Quantification of the gnostic sensitivity via measurement of the vibration threshold and of finger tip sensation].

OBJECTIVE: Quantification of vibration perception and fingertip sensation in routine neurological examination. SETTING: Neurological Clinic, University Hospital, Groningen, the Netherlands. DESIGN: Prospective, controlled investigation. METHODS: Vibration perception and fingertip sensation were quantified in a large group of normal control persons of various ages and in neurological patients and compared with the usual sensory tests at routine neurological examination. The vibration perception limit was measured with a biothesiometer without accelerometer, the fingertip sensation with a device for two-point discrimination slightly modified according to Renfrew ('Renfrew meter'). Concordance of the tests was studied by calculating kappa values. RESULTS: The normal values of both sensory qualities had a log-normal distribution and increased with age. The values obtained with the Renfrew meter correlated well with those of the two-point discrimination and stereognosis but were systematically higher than those indicated by Renfrew. CONCLUSION: Both methods appear useful at routine neurological examination if certain measuring precautions are taken.

Adolescent↗

PubMed1992

Mechano-electrical transducer currents in hair cells of the cultured neonatal mouse cochlea.

The first step towards the generation of the receptor potential in hair cells is the gating of the transducer channels and subsequent flow of transducer current, induced by deflection of the stereocilia. We describe properties of the transducer current in outer hair cells of neonatal mice. Less extensive observations on inner hair cells suggest that their transducer currents have similar characteristics. The hair bundles were stimulated by force from a fluid jet. The transducer currents in outer hair cells are the largest found so far in any hair cell, with a chord conductance of up to 9.2 nS at -84 mV. The transfer function suggests that the channel has at least two closed states and one open state. The permeabilities for sodium, potassium and caesium are similar, consistent with the channel being a fairly non-selective cation channel. At negative potentials the currents adapt in most cells, although never as completely as in hair cells of lower vertebrates. If the unit conductance of the transducer channel is similar to that of the turtle's auditory hair cells (100 pS), then there are about 90 channels per hair bundle, or one channel between every pair of adjacent stereocilia in neighbouring rows.

Animals↗

PubMed1990

Potassium currents in inner hair cells isolated from the guinea-pig cochlea.

1. Inner hair cells were mechanically isolated from the apical, low-frequency region of the guinea-pig cochlea and maintained by superfusion with tissue-culture medium. Membrane currents were studied under voltage clamp, using the whole-cell recording mode of the patch-clamp technique. 2. The cells were studied mostly at 35-38 degrees C to obtain realistic kinetics of the currents, relevant to the functioning of these cells in vivo. 3. Isolated inner hair cells had resting potentials of about -65 mV. Depolarizing voltage steps from a holding potential of about -80 mV resulted in large time- and voltage-dependent outward currents. Hyperpolarizing voltage steps from the same holding potential only showed a small leakage conductance of 0.5-2.5 nS. 4. On repolarization to different membrane potentials, the tail currents reversed around -75 mV. This indicates that the outward currents were mainly carried by potassium ions. 5. Pharmacological dissection of the currents provided evidence for two different potassium conductances. The largest conductance had extremely fast kinetics. Its principal time constant of activation was about 0.15-0.35 ms, the faster values being obtained for larger depolarizations. This fast potassium conductance was blocked by 25 mM-tetraethylammonium chloride in the bath. 6. A smaller, slow potassium conductance, with principal time constants of activation of 2-10 ms (speeding up with depolarization), was blocked by 10-15 mM-4-aminopyridine in the patch pipette. 7. Both potassium conductances were activated over the membrane potential range of about -60 to -20 mV. This is approximately the same as the range of the receptor potential measured in vivo. Therefore these conductances should influence the properties of the receptor potential in inner hair cells. 8. Current injection experiments showed two main effects of the potassium conductances: (a) a non-linearity in the voltage-current relationships; (b) a strongly damped oscillation of the membrane potential in response to a large step of outward current. This oscillatory behaviour is caused by the fast potassium conductance.

4-Aminopyridine↗