Calcium-dependent Current Research Articles

Spike aftercurrents in R15 of Aplysia: their relationship to slow inward current and calcium influx.

Spikes in the bursting neuron, R15, are followed by depolarizing afterpotentials (35) and often by delayed hyperpolarizing afterpotentials as well. Placing the cell in a voltage clamp after a spike allows measurement of the depolarizing aftercurrent (DAC) and hyperpolarizing aftercurrent (HAC) that underlie the afterpotentials. Subthreshold depolarizations give rise to small DACs and HACs. The DAC and the slow inward current (SIC) of R15 are reduced or blocked in a similar manner by many experimental manipulations, e.g., application of dopamine, zero-calcium seawater, zero-sodium seawater, or calcium-channel blockers (Mn2+ and La3+), or cooling the cell from 21-22 degrees C to 10 degrees C. Neither the DAC nor the SIC were blocked by tetrodotoxin (100 uM) and neither was sensitive to altered extracellular potassium. Both the DAC and SIC become larger as the holding potential of the cell is progressively depolarized from -70 to -40 mV. DACs are sensitive to the injection of intracellular calcium chelators (EGTA (ethylene glycol-bis(beta-aminoethyl ether)-N,N1-tetraacetic acid) or EDTA [ethylenedinitrilo)tetraacetic acid]. DAC amplitude is approximately 90% reduced by intracellular EGTA concentration near 1mM. In contrast, the SIC is unchanged or much less affected by the calcium buffers. DACs are also more sensitive to low (1 mM) extracellular calcium than is the SIC. The HAC is also a calcium-dependent current. It is blocked by any experimental manipulation reducing calcium influx or intracellular calcium accumulation, i.e., reduced extracellular calcium, calcium-channel blockers, or intracellular EGTA. We suggest that the DAC and the SIC are carried by the same conductance mechanism. In the case of the DAC, the conductance might be activated by a rise in intracellular calcium activity accompanying the spike and, in the case of the SIC, depolarization per se may be the most important activating condition.

Effect of diltiazem on calcium transport and development of tension in heart muscle

Diltiazem relaxes vascular smooth muscle at concentrations as low as 10 −8 M . These concentrations do not affect either the action potential or the contractile activity of cardiac muscle. In this study the effect of diltiazem on excitation-contraction coupling processes in frog ventricular muscle was investigated. Diltiazem had little effect on the cardiac action potential or tension at concentrations less than 10 −7 M . At higher concentrations (10 −5 M ), the action potential shortened, the overshoot potential decreased, twitch tension was reduced, and the resting potential was unaffected. Potassium-chloride induced contractures were reduced by about 60 percent, suggesting that the effect of diltiazem was not due to the alteration in the action potential duration. In chemically skinned myocardium, the sensitivity of the myofilaments to calcium (Ca 2+ ) was not altered by diltiazem. The effects of diltiazem could be reversed in part either by increasing extracellular calcium or by adding epinephrine. Diltiazem (2 × 10 −6 M ) suppressed the tension-voltage relation under voltage clamp conditions. However, at even higher concentrations, significant tension could still be generated. Diltiazem reduced the slow inward current somewhat, but no direct relation could be found between suppression of the calcium-dependent inward current (I si ) and inhibition of tension. Diltiazem increased the late outward current but had no effect on the inwardly rectifying potassium (K + ) channel. In sodium (Na + )-depleted ventricular preparations, the tension-voltage relation, which reflects the voltage dependence of I si , is completely suppressed by diltiazem (2 × 10 −5 M ). These results suggest that diltiazem in large doses suppresses tension either by inhibition of a slowly inactivating Ca 2+ current or by modifying an undescribed step in excitation-contraction coupling processes.

Cytoplasmic alkalization reduces calcium buffering in molluscan central neurons

The effect of raised cytoplasmic pH (pHi) on intracellular concentration ([Ca2+]i) transients following calcium influx during membrane depolarization was studied in identified neurons in the abdominal ganglion of Aplysia californica. The pHi was monitored with pH-sensitive microelectrodes. Sea water containing 15 mM NH4Cl at pH 7.7 elevated pHi about 0.35 pH units from the normal level of 7.17. These cells have an estimated buffering power of about 60 mM/pH unit. Calcium influx was elicited by depolarizing pulses under voltage clamp and [Ca2+]i transients were monitored with the photoprotein aequorin or the metallochromic dye arsenazo III. Aequorin photo-emissions increased by 21--131% (mean, 48%) and arsenazo III absorbance changes accompanying depolarization increased by 9--33% (mean, 20%) after 30 min in NH4+, corresponding roughly to a 14% increase in [Ca2+]i transients. Calcium-dependent potassium tail currents following a depolarizing pulse were somewhat slower and 4--91% (mean, 38%) large in NH4+. The magnitude and time- and voltage-dependence of the membrane calcium conductance was studied using calcium tail currents following depolarizing pulses. The calcium current was unaffected by NH4+, so the enhanced [Ca2+]i transients must reflect reduced calcium buffering at high pHi. Either reduced cytoplasmic calcium binding or slowed active extrusion of calcium may be responsible for this effect.

Generation of a fixed motor pattern

1. Electrical properties of neurons involved in the production of the pyloric output pattern of the stomatogastric ganglion of the crab,Cancer pagurus, as well as characteristics of synaptic connections of these neurons were investigated. 2. The bursting discharge activity of the medium sized pyloric dilator neurons (m-PD) is found to be of endogenous origin. The ionic mechanism of the bursting activity is suggested to consist of a coactivated sodium-calcium inward current opposed by a calcium dependent potassium outward current. The small-sized pyloric dilators (s-PD), lateral pyloric (LP) and pyloric neurons (PY) also have the capability to discharge spontaneously. 3. Low-pass properties of the neurite segment connecting the spike trigger zone to the soma is indicated. This seems to account for the small amplitudes of action potentials recorded usually at the soma. 4. The values of different electronic length parameters suggest differences in the collateral properties. The effect of passive collateral properties on synaptic parameters is discussed. 5. The pyloric neurons can be driven by excitatory synaptic (EPSP) input. With increased stimulus strength additional input fibres are recruited. Increasing the stimulus frequency augments the burst-frequency and spike-frequency per burst, however, the number of spikes per burst stays relatively constant. 6. The time-course and amplitude of EPSPs recorded from pacemaker neurons are reduced shortly after the burst. EPSP's late in the interburst phase however were prolonged. Active membrane processes underlying the pacemaker activity appear to be responsible for these modifications of synaptic parameters. 7. Hyperpolarizing current injection reduces EPSP-amplitude and shortens the time-course of EPSPs. Since no anomalous conductance changes of the soma membrane were found, it is presumed that the anomalous conductance changes occur at the collateral membrane. 8. Intraganglionic chemical synaptic interaction of the pyloric neurons is exclusively inhibitory. The IPSPs for the various neurons show characteristic differences in delay, amplitude and time-course. This may contribute to the sequencing of the LP-PY discharge in the pyloric pattern. The ionic mechanism of the postsynaptic inhibition of pyloric neurons is found to be chlorid-dependent. 9. It is concluded that the endogenous discharge properties of the pyloric neurons produces a basic level of activity and that the differences in passive and active membrane properties and their effect on synaptic parameters contributes to the generation of the pyloric output pattern.