Send to

Choose Destination
J Physiol. 1994 Dec 1;481 ( Pt 2):251-71.

Regulation of intracellular calcium and calcium buffering properties of rat isolated neurohypophysial nerve endings.

Author information

Department of Physiology, University of Michigan, Ann Arbor 48109, USA.


1. Electrophysiological measurements of Ca2+ influx using patch clamp methodology were combined with fluorescent monitoring of the free intracellular calcium concentration ([Ca2+]i) to determine mechanisms of Ca2+ regulation in isolated nerve endings from the rat neurohypophysis. 2. Application of step depolarizations under voltage clamp resulted in voltage-dependent calcium influx (ICa) and increase in the [Ca2+]i. The increase in [Ca2+]i was proportional to the time-integrated ICa for low calcium loads but approached an asymptote of [Ca2+]i at large Ca2+ loads. These data indicate the presence of two distinct rapid Ca2+ buffering mechanisms. 3. Dialysis of fura-2, which competes for Ca2+ binding with the endogenous Ca2+ buffers, reduced the amplitude and increased the duration of the step depolarization-evoked Ca2+ transients. More than 99% of Ca2+ influx at low Ca2+ loads is immediately buffered by this endogenous buffer component, which probably consists of intracellular Ca2+ binding proteins. 4. The capacity of the endogenous buffer for binding Ca2+ remained stable during 300 s of dialysis of the nerve endings. These properties indicated that this Ca2+ buffer component was either immobile or of high molecular weight and slowly diffusible. 5. In the presence of large Ca2+ loads a second distinct Ca2+ buffer mechanism was resolved which limited increases in [Ca2+]i to approximately 600 nM. This Ca2+ buffer exhibited high capacity but low affinity for Ca2+ and its presence resulted in a loss of proportionality between the integrated ICa and the increase in [Ca2+]i. This buffering mechanism was sensitive to the mitochondrial Ca2+ uptake inhibitor Ruthenium Red. 6. Basal [Ca2+]i, depolarization-induced changes in [Ca2+]i and recovery of [Ca2+]i to resting levels following an induced increase in [Ca2+]i were unaffected by thapsigargin and cyclopiazonic acid, specific inhibitors of intracellular Ca(2+)-ATPases. Caffeine and ryanodine were also without effect on Ca2+ regulation. 7. Evoked increases in [Ca2+]i, as well as rates of recovery from a Ca2+ load, were unaffected by the extracellular [Na+], suggesting a minimal role for Na(+)-Ca2+ exchange in Ca2+ regulation in these nerve endings. 8. Application of repetitive step depolarizations for a constant period of stimulation resulted in a proportional frequency (up to 40 Hz)-dependent increase in [Ca2+]i. On the other hand, for a constant number of stimuli a reduction in the [Ca2+]i. On the other hand, for a constant number of stimuli a reduction in the [Ca2+]i increase per impulse was observed at higher frequencies.(ABSTRACT TRUNCATED AT 250 WORDS).

[Indexed for MEDLINE]
Free PMC Article

Supplemental Content

Full text links

Icon for Wiley Icon for PubMed Central
Loading ...
Support Center