RESUMO
We describe how potential artifacts (due to solution composition, buffering capacity of the bathing medium, size of the skinned fiber preparation, permeability of the sarcoplasmic reticulum (SR) vesicles, and proper Kd for Ca2+ of the fluorescent indicator used to measure Ca2+ transport can be avoided in order to determine the effects of inorganic phosphate (Pi, or any other ion) on m,aximum Ca2+ activated force (Fmax) and Ca2+ sensitivity of skinned cardiac muscle fibers, and Ca2+ -ATPase activity and uptake properties of isolated cardiac SR-enriched vesicles. To maintain the ionic strengh of the bathing medium constant when adding Pi, other ions must be removed. We found that because some salts have a depressant effect on Fmax independent of increaqsed ionic strength (e.g. KCl) while others do not (e.g. Na-acetate), the salt used to adjust ionic strength influences the measured depressant effect of Pi on Fmax. The sensitivity to Ca2+ of the contractile apparatus depends on the sum of the [Na+] and [K+] in the bething medium. However, we found that the effect of Pi on Ca2+ sensitivity was not significantly influenced by the small changes in the sum of [Na+] and [K+] that were associated with the addition of Pi. The skinned fiber preparations were approximately cylindrical bundles with diameters ranging between 100 and 250 um. We found that the effect of Pi on Fmax was not influenced by diffusion limitations over this range of bundle diameters. The pH buffering capacity of the bathing solution affects Fmax at pH 6.6. We found that the buffering effect of Pi can influence the mechanical response of skinned fibers independent of a Pi on the contractile apparatus when the buffering capacity of the control solution is low. When the Ca2+-ATPase of isolated SR vesicles is activated by Ca2+ and MgATP, the vesicles accumulate Ca2+. Unless the vesicles are permeabilized with a Ca2+ ionophore (ionomycin) and the pH adequately buffered, maximum ATPase activity will be underestimated, the broadness of the curve relating Ca2+ - ATPase rate overestimated, and the sensitivity to Pi overestimated. The ionic milieu of isolated SR vesicles changes the apparent dissociation constant (Kd) of Ca2+-Fura-2, a fluorescent dye used to quantify SR Ca2+ transport rates. In order to accurately measure the inhibitory effect of Pi on Ca2+ uptake, the influence of Pi on the Kd of Fura-2 for Ca2+ must be taken into account