The composition of the luminal fluid in an isolated posterior canal was determined after luminal perfusion with an endolymph-like solution (Ferrary et al., Am. J. Physiol., 22, F182, 1989; J. Physiol., 461, 451, 1993). The basolateral solution was a perilymph-like medium of nonmal osmolarity (240 mOsm/l) during a 1st 10 min period, and was switched during a 2nd period to a new solution of reduced (by decreasing NaCl, 105 mOsm/l, n=6) or increased (by adding Na gluconate, 550 mOsm/l, n=7) osmolarity. Five minutes after the basolateral application of hypo-osmotic solution, [Na], [K] and [Cl] were virtually unchanged but they decreased at 10 min so that the calculated osmolarity of the luminal fluid ([Na]+[K])*2) was 249 ± 0.97 and 224 ± 3.8 mOsm/l (mean ± S.E.M.) in iso- and hypo-osmotic conditions, respectively. On the opposite, 5 min after the application of hyperosmotic basolateral solution, the increase in [Na] was higher than in iso-osmotic conditions, whereas no modification in [K] and [Cl] was observed. At 10 min, the increase in [Na] was more pronounced and [K] slightly decreased, while [Cl] remained stable. MEPSPs and spikes were recorded from the posterior nerve in the isolated intact frog labyrinth superfused with a perilymph-like solution (240 mOsm/l), as well as in solutions where NaCl was either reduced or increased to a final osmolarity of 105-420 mOsm/l. Altematively, the control Ringer was made hypertonic by adding saccarose (550 mOsm/l) or variable amounts of Na gluconate so as to obtain a final osmolarity in the 380-550 mOsm/l range. Hypotonicity decreased, while hypertonicity increased mEPSP and spike frequencies. MEPSP rate was determined by means of signal processing techniques (Rossi et al., J. Gen. Physiol., 24, 303, 1989; J. Physiol., 478., 17, 1994) in control solution and after 2-3 min following the bath immission of the test medium. Statistical analysis of the records shows that: 1) mEPSP rate increased linearly with osmolarity; 2) mEPSP size histograms were well described by single lognormal functions and time interval distributions proved to be monoexponential either in control or in test solutions; 3) the effect of hypertonicity disappeared in a Ca-free medium. The mEPSP waveform was also independently evaluated: single events were aligned and averaged and the mean waveforms were fitted by a modified y-distribution function. MEPSP peak size depended on external [Na] but was insensitive to saccarose. The increase in mEPSP and spike rates, observed during hyperosmotic treatments, indicates that the hair cell transduction current is not significantly modified by changes in osmolarity. Our results also suggest that: 1) the basic mechanism of transmitter release remains asynchronous and uniquantal irrespective of the osmotic treatments; 2) no modifications of the quantum size are produced by tonicity; 3) the osmotic effects depend on extemal calcium.
EFFECTS OF HYPO- AND HYPERTONIC SOLUTIONS ON ENDOLYMPH COMPOSITI0N AND mEPSPS AND SPIKE RATES IN THE ISOLATED FROG LABYRINTH
ROSSI, Marialisa;MARTINI, Marta;PELUCCHI, Bruna;
1995
Abstract
The composition of the luminal fluid in an isolated posterior canal was determined after luminal perfusion with an endolymph-like solution (Ferrary et al., Am. J. Physiol., 22, F182, 1989; J. Physiol., 461, 451, 1993). The basolateral solution was a perilymph-like medium of nonmal osmolarity (240 mOsm/l) during a 1st 10 min period, and was switched during a 2nd period to a new solution of reduced (by decreasing NaCl, 105 mOsm/l, n=6) or increased (by adding Na gluconate, 550 mOsm/l, n=7) osmolarity. Five minutes after the basolateral application of hypo-osmotic solution, [Na], [K] and [Cl] were virtually unchanged but they decreased at 10 min so that the calculated osmolarity of the luminal fluid ([Na]+[K])*2) was 249 ± 0.97 and 224 ± 3.8 mOsm/l (mean ± S.E.M.) in iso- and hypo-osmotic conditions, respectively. On the opposite, 5 min after the application of hyperosmotic basolateral solution, the increase in [Na] was higher than in iso-osmotic conditions, whereas no modification in [K] and [Cl] was observed. At 10 min, the increase in [Na] was more pronounced and [K] slightly decreased, while [Cl] remained stable. MEPSPs and spikes were recorded from the posterior nerve in the isolated intact frog labyrinth superfused with a perilymph-like solution (240 mOsm/l), as well as in solutions where NaCl was either reduced or increased to a final osmolarity of 105-420 mOsm/l. Altematively, the control Ringer was made hypertonic by adding saccarose (550 mOsm/l) or variable amounts of Na gluconate so as to obtain a final osmolarity in the 380-550 mOsm/l range. Hypotonicity decreased, while hypertonicity increased mEPSP and spike frequencies. MEPSP rate was determined by means of signal processing techniques (Rossi et al., J. Gen. Physiol., 24, 303, 1989; J. Physiol., 478., 17, 1994) in control solution and after 2-3 min following the bath immission of the test medium. Statistical analysis of the records shows that: 1) mEPSP rate increased linearly with osmolarity; 2) mEPSP size histograms were well described by single lognormal functions and time interval distributions proved to be monoexponential either in control or in test solutions; 3) the effect of hypertonicity disappeared in a Ca-free medium. The mEPSP waveform was also independently evaluated: single events were aligned and averaged and the mean waveforms were fitted by a modified y-distribution function. MEPSP peak size depended on external [Na] but was insensitive to saccarose. The increase in mEPSP and spike rates, observed during hyperosmotic treatments, indicates that the hair cell transduction current is not significantly modified by changes in osmolarity. Our results also suggest that: 1) the basic mechanism of transmitter release remains asynchronous and uniquantal irrespective of the osmotic treatments; 2) no modifications of the quantum size are produced by tonicity; 3) the osmotic effects depend on extemal calcium.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
