title: Dynamics of intracellular free calcium concentration in the presynaptic arbors of individual barnacle photoreceptors.
abstract: At photoreceptor synapses, transmitter release is continuous and graded. At this type of synapse, the control of presynaptic [Ca2+]i and calcium's role in releasing transmitter might be different than at terminals invaded by all-or-none action potentials. To examine this possibility, we measured the spatial and temporal changes of [Ca2+]i in response to depolarization of individual photoreceptor terminals of the barnacle Balanus nubilus, which had been injected with the Ca2+ indicator Fura-2. Depolarizing pulses produced voltage-dependent Ca2+ entry that was confined to the tips of the arbor where the release sites are located. At increasing distances from the tips, the rate of [Ca2+]i increase was slower and the peak [Ca2+]i occurred later, suggesting that Ca2+ entered the tips and diffused back into the larger processes of the arbor. Consistent with this result, a stable gradient of [Ca2+]i was observed at maintained depolarizations, with the highest values at the tips of the arbor. Removal of external Na+ did not affect the time course of Ca2+ decline in the terminal, indicating that Na+/Ca2+ exchange was not the primary mechanism for restoring [Ca2+]i to basal levels. Computer simulations, assuming only Ca2+ entry at the arbor's tips and diffusion of Ca2+ away from the entry site, qualitatively reproduced these observations. The threshold for Ca2+ entry was near -60 mV, and entry was maintained during prolonged depolarizations, in agreement with previous experiments showing that Ca2+ channels in the terminal region do not inactivate. The time course of the measured [Ca2+]i change in the terminal paralleled voltage changes due to a Ca(2+)-activated K+ conductance, which senses [Ca2+]i just under the membrane. This parallelism is expected since the release sites are located on processes of small-enough diameter to permit radial equilibration of [Ca2+]i within the time course of physiological voltage changes. Therefore, the optical measurements reflect the mean level of [Ca2+]i under the membrane. Whether this mean concentration is also the value at the sites that trigger exocytosis will depend on how close the Ca2+ channels are to these sites.
authors.affiliation: Department of Physiology, University of North Carolina, Chapel Hill 27599.
authors.forename: J C
authors.initials: JC
authors.lastname: Callaway
authors.forename: N
authors.initials: N
authors.lastname: Lasser-Ross
authors.forename: A E
authors.initials: AE
authors.lastname: Stuart
authors.forename: W N
authors.initials: WN
authors.lastname: Ross
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D016257:Fura-2; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D010786:Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D013050:Spectrometry, Fluorescence; D013569:Synapses / Q000502:physiology*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D001468:Thoracica; D013997:Time Factors
chemical_list: D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D002118:Calcium; D016257:Fura-2