Molluscan Central Neurons

The effect of raised cytoplasmic pH (pHi) on intracellular calcium 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 NH 4C1 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~o (mean, 4 8 ~ ) a n d arsenazo III absorbance changes accompanying depolarization increased by 9-33 ~ (mean, 20 ~ ) after 30 min in N H C , 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 ~ ) larger in NH4 ÷. The magnitude and timeand 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.