Creatine Kinase and Its Role in Distributing Energy in Rod Photoreceptors in Light and Darkness

Abstract

Purpose: : To develop an understanding of how energy is distributed within photoreceptors. In darkness photoreceptors consume energy primarily in the inner segment and synaptic terminal. In light they consume energy primarily in the outer segment. Creatine phosphate (PCr) is better than ATP as a transporter of high energy phosphate. PCr has greater mobility and it is less likely than ATP to be bound to proteins or to be hydrolyzed before reaching distant sites with high energy demand. Ubiquitous mitochondrial creatine kinase (uMtCK) synthesizes PCr from ATP made by mitochondria. Cytosolic creatine kinase (B-CK) transfers the high energy phosphate from PCr back to ATP at sites where ATP is needed. This study defines the distribution of creatine kinases in retina and it initiates an investigation of their physiological functions in photoreceptors.

Methods: : Creatine kinase isoforms were localized using western blots of serial sections of rat retina. The distribution was confirmed by creatine kinase activity assays. To evaluate the influence of creatine kinase activity on photoreceptor function we used fluorodintrobenzene (FDNB), a creatine kinase inhibitor. Light-stimulated rhodopsin phosphorylation was determined by mass spectrometry. Dark current was evaluated by suction electrode recordings.

Results: : uMtCK is in the ellipsoid region of the photoreceptor layer. B-CK levels in the Inner segment layer are >10X higher than in the outer segment layer. B-CK is most abundant in the photoreceptor synapse layer. In darkness 10 µM FDNB causes a ~50% decrease in dark current. In light 10 µM FDNB does not affect light-stimulated rhodopsin phosphorylation in intact mouse retinas.

Conclusions: : Our findings suggest the following hypothesis:Dark-adapted rods: PCr carries energy from mitochondria to energy consuming sites within the inner segments of rods.Light-adapted rods: ATP is the predominant carrier of energy to the outer segment. Additional studies are underway to test the validity of this hypothesis.

Keywords: metabolism • photoreceptors • mitochondria