Intracellular calcium measurements with PVC-resin Ca-selective microelectrodes in frog proximal tubules and sartorius muscle fibers.

To measure ion activity of intracellular calcium, (Ca)i, we manufactured a Ca2(+)-selective microelectrode based on a neutral carrier, ETH-1001, with or without polyvinylchloride column (PVC-resin). Before and after cell impalements (n = 7), PVC-resin Ca2(+)-selective microelectrodes exhibited Nernstian slopes in the range of p(Ca) from 3 to 7 (the mean slope constant: 29.1 and 29.4 mV/p(Ca), respectively) and sub-Nernstian slopes between p(Ca) = 7 and 8 (21.6 and 21.1 mV/p(Ca]. Individual detection limits of PVC-resin microelectrodes averaged p(Ca) = 8.5 before and 8.3 after cell impalements. In contrast, the non-PVC-resin microelectrode exhibited a poor response between p(Ca) = 7 and 8 especially after cell impalements. Normal values obtained with the PVC-resin microelectrode for (Ca)i of proximal tubule cells and resting sartorius muscle fibers in the bullfrog averaged 18 +/- 2 (n = 10) and 12 +/- 2 (n = 7) nM (mean +/- S.E., number of observations), respectively. We conclude that 1) the PVC-resin microelectrode is a useful tool for measuring intracellular ion activities, and 2) the values of intracellular Ca2+ activity of the proximal tubule and skeletal muscle fibers measured with this microelectrode are in a lower range among the various values reported before with the other methods.