| dc.description | Calcium-activated chloride channels are present in cone photoreceptors but their physiological role has not clearly been defined. This dissertation characterizes the kinetics and regulation of Ca2+ -activated Cl- currents (ICl(Ca)) in salamander cone photoreceptors and provides evidence for their functional role in the retina. ICl(Ca), sensitive to the Cl channel blocker NPPB, activates in response to depolarization-evoked Ca2+ influx. The slow and complex deactivation kinetics of Ca2+-activated Cl- tail currents following repolarization is considered to reflect the changing submembrane concentration of intracellular calcium ([Ca2+]i). The dihydropyridine Ca channel agonist BayK increased ICl(Ca), demonstrating the importance of Ca 2+ entry through L-type Ca channels for Cl(Ca) channel activation. The deactivation kinetics of ICl(Ca) were resistant to manipulations of Ca2+ uptake and release from Ca2+ stores. The sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) pump inhibitors, cyclopiazonic acid and thapsigargin, had no significant effect on tail current kinetics. IP3 and the Ca2+-induced Ca 2+ release (CICR) modulators, caffeine and ryanodine, also had no discernable effect on the tail current. However, the submembrane [Ca 2+]i dominated deactivation kinetics of ICl(Ca) were altered by increased [Ca2+]i when modulating agents of Ca2+ stores were combined with repetitive depolarizations to induce Ca2+-loading. Under these conditions, ICl(Ca) became strongly inhibited. Thus, the normal activation and deactivation of ICl(Ca) appears to be dominated by fast Ca2+ diffusion and buffering, but stores provide essential regulation of [Ca 2+]i when Ca2+ influx is very high. High [Ca2+]i-induced inhibition of ICl(Ca), which involves dephosphorylation mechanisms within the cell, may reflect fast inhibition of Ca channels, followed by a slower inhibition of Cl(Ca) channels. In darkness, when intracellular Ca2+ levels are highest within cone photoreceptors, ICl(Ca) may prevent strong depolarizations; however, in overloaded Ca2+ conditions, Ca and Cl(Ca) channel inhibition likely represents an effective protective mechanism for the cells. | en_US |
