Flagellar quiescence and transience of inactivation induced by rapid pH drop

The effects of rapid pH drop on the flagellar movement of reactivated sea urchin sperm were studied by video microscopy and by a newly developed pH jump method. Triton-demembranated sperm were reactivated in a thin layer of the reactivation medium containing ATP and potassium acetate and supported by a ring-shaped Millipore filter stuck to the lower surface of a supported coverslip. The pH of the medium was lowered rapidly by dissolving acetic acid vapor abruptly introduced into a gap between the cover and slide. Flagellar beating ceased immediately when the pH of the reactivation medium was lowered. At least two types of cessation were distinguished: 1) "instantaneous" cessation in a bent form closely resembling those characteristic of steady-state beating before pH drop (waveform freeze), and 2) flagellar quiescence in a cane-shaped form resembling those characteristic of Ca-induced quiescence (cane-shaped quiescence). The flagellum again began beating if the pH was raised to normal but eventually was disintegrated by tubule sliding if the pH was left lowered. Field-by-field analysis of the transient movement of flagella becoming quiescent upon pH drop demonstrated that the proximal bend of the cane-shaped form corresponded to the principal bend of the steady-state beating in some flagella, but in others, to the reverse bend. These observations indicate that low pHs affect flagellar beating by interfering with sliding-bending conversion by a mechanism different from that previously reported.