Biomedical subjects

A B Ribera

Publications and source records attributed to A B Ribera.

28 records · Page 2Linked to original sources

PubMed1991

Differentiation of delayed rectifier potassium current in embryonic amphibian myocytes.

The developmentally regulated expression of prolonged outward potassium currents influences the extent to which sustained inward currents contribute to the action potential at early stages of differentiation. In amphibian spinal neurons, the long duration and calcium dependence of the embryonic action potential and the amount of calcium influx are largely determined by the extent of maturation of the delayed rectifier potassium current (IKv). We have undertaken a parallel study of differentiation of myocytes, in which action potentials are brief and sodium-dependent even at early stages. The early expression of electrical excitability in embryonic amphibian myocytes growing in culture has been examined previously using intracellular voltage recording techniques. The membrane exhibits a delayed rectification in response to depolarization at times earlier than those at which impulses can first be generated. We have examined the differentiation of this outward current in embryonic myocytes developing in vitro, using whole cell voltage clamp. IKv is initially absent. When first recorded it is small and slowly activating but undergoes sixfold increases in both density and rate of activation during the first day in culture. This maturation is dependent upon transcription, and both rate and density are influenced by the presence of other cell types. The large amplitude of the outward delayed rectifier prevents expression of long duration action potentials.

Animals↗

PubMed1990

A potassium channel gene is expressed at neural induction.

Voltage-dependent potassium currents exhibit specific time tables of functional differentiation and regulate the development of action potentials in amphibian spinal neurons. A Xenopus nucleotide sequence (XSha2) encoding a potassium current has been isolated by homology screening with the Drosophila Shaker gene. Functional expression in oocytes identifies it as a delayed rectifier. Southern analysis suggests that XSha2 is a member of a family of highly related genes. XSha2 is expressed in the nervous system but is not detectable in skeletal muscle. Transcripts are apparent at the neural fold stage, and subsequent levels parallel those of the neural marker N-CAM. Thus molecular events required for the establishment of electrical excitability in the vertebrate embryo occur early during neurogenesis.

Amino Acid Sequence↗

PubMed1990

Differentiation of IKA in amphibian spinal neurons.

We have examined the development of an inactivating outward current in embryonic amphibian neurons differentiating in culture. On the basis of ionic selectivity, voltage dependence of activation and inactivation and pharmacological sensitivity, it is similar to A currents described in other neurons. In these embryonic neurons the A current appears later than other voltage-dependent currents studied previously. Furthermore, its maturation extends to times later than those required for the differentiation of another potassium current, the voltage-dependent delayed rectifier. These changes in A current are consistent with parallel changes in the action potential and excitability of the developing neurons.

Action Potentials↗

PubMed1989

The actions of verapamil at the neuromuscular junction.

1. The actions of the calcium channel blocker verapamil were studied at the neuromuscular junction of the frog Rana pipiens. 2. In the presence of 50 microM verapamil, subthreshold endplate potentials were produced, and the quantal content was reduced by a factor of 3. 3. Verapamil (10-50 microM) also reduced the postjunctional membrane sensitivity as measured by (a) carbachol iontophoresis and (b) miniature endplate potential amplitude. In addition, verapamil had a strong inhibitory effect on the postjunctional membrane response to repetitive iontophoretic application of carbachol. 4. Thus, verapamil has both pre- and postsynaptic actions at the neuromuscular junction.

Action Potentials↗

PubMed1989

A critical period of transcription required for differentiation of the action potential of spinal neurons.

The early development of excitability of amphibian spinal neurons is characterized by a change from a long Ca2(+)-dependent action potential to a brief Na(+)-dependent impulse. The delayed rectifier K+ current plays a major role in this cell autonomous differentiation. Here we show that the maturation of the delayed rectifier current, and hence the action potential, involves a critical period of mRNA synthesis. It is blocked by inhibition of transcription during an early period of development in culture and fails to develop following removal of the inhibitor and resumption of RNA synthesis. However, the development of an inactivating K+ A-current recovers in these neurons, indicating that some programs of neuronal development are affected during this critical period, while others are spared.

Action Potentials↗

PubMed1988

Effects of chronic prednisolone treatment on postjunctional membrane responses to agonist.

Using frogs treated 1 month with prednisolone, we studied responses of the muscle postjunctional membrane to acetylcholine and carbamylcholine. In control muscles following a bath application of 3 microM neostigmine or 30 microM pyridostigmine, there was an increased depolarization response to iontophoretically applied acetylcholine or carbamylcholine. Results with prednisolone-treated frog muscle were essentially identical. In some fibers of prednisolone-treated muscle there was a decrementing response to repetitive iontophoretic application of carbamylcholine. Prolonged administration of prednisolone may accelerate desensitization of acetylcholine receptors during repetitive application of agonist.

Acetylcholine↗

PubMed1988

Development of voltage-dependent calcium, sodium, and potassium currents in Xenopus spinal neurons.

Action potentials of embryonic nerve and muscle cells often have a different ionic dependence and longer duration than those of mature cells. The action potential of spinal cord neurons from Xenopus laevis exhibits a prominent calcium component at early stages of development that diminishes with age as the impulse becomes principally sodium dependent. Whole-cell voltage-clamp analysis has been undertaken to characterize the changes in membrane currents during development of these neurons in culture. Four voltage-dependent currents of cells were identified and examined during the first day in vitro, when most of the change in the action potential occurs. There are no changes in the peak density of the calcium current (ICa), its voltage dependence, or time to half-maximal activation; a small increase in inactivation is apparent. The major change in sodium current (INa) is a 2-fold increase in its density. In addition, more subtle changes in the kinetics of the macroscopic sodium current were noted. The peak density of voltage-dependent potassium current (IKv) increases 3-fold, and this current becomes activated almost twice as fast. No changes were noted in the extent of its inactivation. The calcium-dependent potassium current (IKc) consists of an inactivating and a sustained component. The former increases 2-fold in peak current density, and the latter increases similarly at less depolarized voltages. The changes in these currents contribute to the decrease in duration and the change in ionic dependence of the impulse.

Action Potentials↗

PubMed1987

Ruthenium red reduces acetylcholine sensitivity and increases desensitization at the frog neuromuscular junction.

The actions of Ruthenium Red on the synaptic membrane were studied at the neuromuscular junction of the frog Rana pipiens. Ruthenium Red blocks mitochondrial calcium transport and thus is expected to elevate the intracellular calcium level [Alnaes and Rahamimoff (1975) J. Physiol., Lond. 248, 285-306]. The postjunctional membrane sensitivity was reduced by Ruthenium Red as determined by (a) the amplitude of the miniature endplate potentials, (b) iontophoretic application of carbachol, (c) microperfusion of carbachol. Desensitization was assayed by measuring the decline in the amplitude of the postsynaptic depolarization produced during repetitive iontophoretic application of carbachol. Following Ruthenium Red treatment desensitization was increased. This action of Ruthenium Red was enhanced by raising the extracellular calcium concentration from 1.8 to 10 mM.

Acetylcholine↗

PubMed1987

Both barium and calcium activate neuronal potassium currents.

Amphibian spinal neurons in culture possess both rapidly inactivating and sustained calcium-dependent potassium current components, similar to those described for other cells. Divalent cation-dependent whole-cell outward currents were isolated by subtracting the voltage-dependent potassium currents recorded from Xenopus laevis neurons in the presence of impermeant cadmium (100-500 microM) from the currents produced without cadmium but in the presence of permeant divalent cations (50-100 microM). These concentrations of permeant ions were low enough to avoid contamination by macroscopic inward currents through calcium channels. Calcium-dependent potassium currents were reduced by 1 microM tetraethylammonium. These currents can also be activated by barium or strontium. Barium as well as calcium activated outward currents in young neurons (6-8 hr) and in relatively mature neurons (19-26 hr in vitro). However, barium influx appeared to suppress the sustained voltage-dependent potassium current in most cells. Barium also activated at least one class of potassium channels observed in excised membrane patches, while blocking others. The blocking action may have masked and hindered detection of the stimulatory action of barium in other systems.

Animals↗

PubMed1986

Effects of chlorpromazine and phencyclidine on mouse C2 acetylcholine receptor kinetics.

Patch-clamp techniques were used to record acetylcholine- (ACh) activated single-channel currents in cell-attached membrane patches from myotubes of the mouse cell line, C2. The effects of the phenothiazine derivative chlorpromazine (CPZ) and of the hallucinogen phencyclidine (PCP) on ACh-activated single-channel properties were studied under conditions where both compounds are positively charged (pH 7.2). The single-channel conductance was unaffected by either CPZ or PCP at concentrations ranging from 10 to 500 nM. 10-200 nM-CPZ and PCP led to shortened mean burst times. CPZ and PCP effects on mean burst times were voltage independent and did not vary in a simple linear manner with concentration. 10-200 nM-CPZ and PCP did not reduce channel opening frequencies, suggesting that the fraction of non-conducting state (occupied, blocked or desensitized) favoured at equilibrium was not significant at these concentrations. On the other hand, concentrations of CPZ and PCP higher than 300 nM did lead to depressed channel opening frequencies. In addition, we observed that, at these concentrations, the shortened burst duration reverses to the longer values found at lower effector concentrations. The effects of CPZ and PCP on ACh-activated single-channel kinetics are interpreted in terms of current models of ACh-receptor structure and conformational transitions.

Acetylcholine↗