Potassium-induced slow motility is partially calcium-dependent in isolated outer hair cells

Low flow rate (0.6 microl/min) administration of high concentration potassium solutions (12.5, 25 and 37.5 mM) was tested for evoking slow-motility length changes in isolated, apical turn, guinea pig outer hair cells (OHCs) (length 65-80 microm; n = 38). Control OHCs (n = 16) showed a flow rate-dependent, reversible, longitudinal shortening of 0.5-3 microm during perfusion with normal saline. Potassium, an effective depolarizing agent for OHCs, induced a concentration-dependent cell shortening of 0.5-13 microm. These cell shape changes were reversible. The magnitude of shortening was significantly (p < 0.01) decreased in a calcium-free incubation medium (n = 8). The velocity of the shortening was 300 nm/s in the first 10 s after application of 37.5 mM K+ in a normal incubation medium and decreased to 100 nm/s during the next 10 s. Corresponding velocities in calcium-free solutions were 100 and 50 nm/s, respectively. K+-induced shortening velocities were not significantly different from control values after 30 s. It appears that K+-induced OHC shortening is sensitive to the calcium content of the incubation medium during the first 10 s. Higher flow rate (1.5 microl/min) administration of K+ makes the velocity and magnitude of slow motility of OHCs insensitive to the absence of calcium. These results highlight the fact that one of the critical technical points in fluid perfusion experiments with isolated OHCs is selecting a safe low flow rate of < 0.6 microl/min. At this perfusion rate, K+-induced OHC shortening is composed of both calcium-sensitive and -insensitive components.