Cytoplasmic calcium buffer capacity determined with Nitr-5 and DM-nitrophen

We have examined intracellular calcium buffer capacity of cytoplasm from the giant axon of the marine invertebrate Myxicola infundibulum by photolytically releasing calcium from 'caged' compounds, while monitoring free calcium, [Ca2+], with Ca-sensing electrodes. In cytoplasm containing intact organelles, two features of the [Ca2+] response were seen upon light exposure: an initial spike from basal [Ca2+], followed by a slower phase recovery. Both the amplitude of the spike in [Ca2+] and the recovery were reduced by removal of MgATP. If organelles were removed from the cytoplasm, light exposure caused only a step-like change in [Ca2+] with no recovery. Apparent buffer capacities (delta bound Ca/delta free Ca) were unaffected by changing pH from 7.0 to 7.5; however, raising basal free calcium above 3 microM significantly reduced this parameter. The buffer capacity measured after the initial spike varied by as much as an order of magnitude from one giant axon to another but averaged approximately 50 in the absence and approximately 100 in the presence of 1 mM MgATP for [Ca2+] below 3 microM.