Barium ions are known to interact with the K+ conductance in several preparations as well as to substitute for Ca2+ in resting and evoked transmitter release. The primary afferent resting activity ìn the frog labyrinth is a Ca2+-dependent process, which is extremely sensitive to the external K level; it seems thus interesting to investigate the effect of Ba2+ on the EPSP and spike discharges recorded from single fibres of the posterior nerve in the isolated preparation. Control recordings were performed in Ringer solution of the following ionic composition (mM): NaCl 116, KCl 2.5, CaCl2 1.8, TrisCl 5. The changes in bathing saline were usually accompanied by appropriate modifications to the NaCI concentration to keep the solution isoosmolar. When BaCl2 was added to a Calcium free Ringer solution the resting activity was preserved; moreover, on increasing Ba2+ concentration (0.9 - 4 mM), an obvious facílitation of the EPSP and spike discharges was observed after 3 - 5 minutes. The spike shape and duration were unaffected by this treatment. In 15 mM Mg2+- Calcium-free Ringer solution the resting activity was completely abolished and no recovery was observed by gradually rising the Ca2+ concentration (1.8 - 18.6 mM). However, when Ba2+ (1.8 mM) was added to the bathing medium the Mg2+ block was fully releaved, independently of the Ca2+ levels, though no facilitatìon was observed. A sìmìlar recovery was not obtaìned in 15 mM Mg2+ - 18.6 mM Ca2+ - 7.5 mM K+ solution, whích rules out the possibility that recovery in spike activity might be simply related to a Ba2+ induced depolarization of the hair cell. Finally, when the resting activity was completely abolished by a 3 - 5 mM CoCl2-Calcium-free Ringer solution, some recovery was obtained in the presence of BaCl2 (1.8 mM), whereas an isosmolar amount of CaCl2 was completely uneffective. The time of recovery increases and facilitation decreases on increasing CoCl2 concentration. It follows that Ba2+ions have a powerful and persistent effect on the transmitter release that may reflect mainly their ability to penetrate the Ca-channel more easily than Ca2+ ions. Ba2+ action might also be related to a depolarizing effect on the hair cell membrane, possibly due to the block of a Ca2+- dependent K+ conductance. On the other hand, Ba2+ ions proved not to interfere with the K+ channels of the myelinated sensory fibres.
Effect of barium ions on the sensory activity of the isolated frog labyrinth
ROSSI, Marialisa;MARTINI, Marta
1984
Abstract
Barium ions are known to interact with the K+ conductance in several preparations as well as to substitute for Ca2+ in resting and evoked transmitter release. The primary afferent resting activity ìn the frog labyrinth is a Ca2+-dependent process, which is extremely sensitive to the external K level; it seems thus interesting to investigate the effect of Ba2+ on the EPSP and spike discharges recorded from single fibres of the posterior nerve in the isolated preparation. Control recordings were performed in Ringer solution of the following ionic composition (mM): NaCl 116, KCl 2.5, CaCl2 1.8, TrisCl 5. The changes in bathing saline were usually accompanied by appropriate modifications to the NaCI concentration to keep the solution isoosmolar. When BaCl2 was added to a Calcium free Ringer solution the resting activity was preserved; moreover, on increasing Ba2+ concentration (0.9 - 4 mM), an obvious facílitation of the EPSP and spike discharges was observed after 3 - 5 minutes. The spike shape and duration were unaffected by this treatment. In 15 mM Mg2+- Calcium-free Ringer solution the resting activity was completely abolished and no recovery was observed by gradually rising the Ca2+ concentration (1.8 - 18.6 mM). However, when Ba2+ (1.8 mM) was added to the bathing medium the Mg2+ block was fully releaved, independently of the Ca2+ levels, though no facilitatìon was observed. A sìmìlar recovery was not obtaìned in 15 mM Mg2+ - 18.6 mM Ca2+ - 7.5 mM K+ solution, whích rules out the possibility that recovery in spike activity might be simply related to a Ba2+ induced depolarization of the hair cell. Finally, when the resting activity was completely abolished by a 3 - 5 mM CoCl2-Calcium-free Ringer solution, some recovery was obtained in the presence of BaCl2 (1.8 mM), whereas an isosmolar amount of CaCl2 was completely uneffective. The time of recovery increases and facilitation decreases on increasing CoCl2 concentration. It follows that Ba2+ions have a powerful and persistent effect on the transmitter release that may reflect mainly their ability to penetrate the Ca-channel more easily than Ca2+ ions. Ba2+ action might also be related to a depolarizing effect on the hair cell membrane, possibly due to the block of a Ca2+- dependent K+ conductance. On the other hand, Ba2+ ions proved not to interfere with the K+ channels of the myelinated sensory fibres.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
