The effect of the phase relationship between the arterial blood gas oscillations and central neural respiratory activity on phrenic motoneurone output in cats.

The aim of the present experiments in artificially ventilated, anesthetized cats was to investigate in which circumstances the timing of the arterial blood gas oscillations within the respiratory cycle can be of importance in determining phrenic motoneurone output. The phase relationship phi was defined as the relative position of the peak of the phrenic bursts within the current continuously measured PaO2 oscillations. It was judged breath by breath whether there was a relationship between phi and neural tidal volume, and neural inspiratory and expiratory duration. Within cats, PETCO2 was kept constant at about 1.5-2% above apneic threshold. It was found that phi indeed partly determined these ventilatory parameters provided the oscillations were large enough. This was evident in normoxia; in moderate hypoxia the influence of phi was demonstrable more easily, i.e. at smaller oscillation amplitudes. In both conditions the effect of phi on neural tidal volume was most pronounced. Neural tidal volume was maximal when peak inspiration coincided with the expiratory trough of the PaO2 oscillations. A 1:1 phase lock between phrenic activity and the ventilatory only occurred when the pump frequency was close to the cats own breathing frequency. Bilateral carotid sinus nerve section abolished the effects of phi.

Effects of metabolic arterial pH changes on medullary ecf pH, csf pH and ventilation in peripherally chemodenervated cats with intact blood-brain barrier.

Brain ecf and csf pH are regulated within narrow limits. In the present study we investigated whether--with an intact blood-brain barrier (bbb)--during acute (isocapnic) metabolic pHa disturbances, brain ecf and csf pH changes occur in the same direction as plasma pH. Experiments were performed in 4 ventilated, vagotomized, and in 17 spontaneously breathing cats. Medullary ecf pH and csf pH were measured with flat pH electrodes. Metabolic pHa disturbances were induced by isocapnic i.v. infusions of HCl and NaHCO3, and by non-isocapnic i.v. bolus injections of NaHCO3. In all cats both sinus nerves were cut. We found that: (1) during acute pHa changes, medullary ecf pH changes rapidly in the same direction despite an intact bbb (no extravasation of Evans Blue); (2) the time courses of the ecf and csf pH responses to a bolus injection of NaHCO3 are quite different: after an initial short (less than 10 sec) decrease, the ecf pH rapidly increases above control, whereas the csf pH shows a rapid and long lasting acidic response; (3) ventilation showed a biphasic response pattern to a NaHCO3 bolus: an initial increase followed by a slow decrease to about control level. It was argued that this response cannot be explained by the observed ecf or csf pH responses alone.

Effects of respiratory and (isocapnic) metabolic arterial acid-base disturbances on medullary extracellular fluid pH and ventilation in cats.

Ventilation is influenced by the brain extracellular fluid (ecf) pH which is sensed by the central chemoreceptors. In the present experiments we have investigated to what extent ventilatory effects of brain ecf pH changes depend on the origin of these pH changes. With this aim we have compared the effects of 'respiratory' (via changes in PaCO2) and 'metabolic' (via isocapnic pHa changes) ecf pH changes on steady state ventilatory activity. Experiments were performed in anaesthetized (both artificially ventilated and spontaneously breathing) cats with cut sinus nerves; medullary surface ecf pH was measured with a glass electrode with a flat pH-sensitive surface. We found that ecf pH changes caused by changes in PaCO2 give rise to greater ventilatory responses than the same ecf pH changes caused by (isocapnic) changes in pHa. Moreover, within the pH ranges measured, isocapnic pHecf-ventilatory response lines at higher PaCO2 are shifted upwards compared with those at lower PaCO2 levels. It was concluded that with the present technique it is impossible to show a unique relation between ecf pH and ventilation.

Some characteristics of tetraphenylphosphonium uptake into Saccharomyces cerevisiae.

The characteristics of the uptake of the lipophilic cation tetraphenylphosphonium (TPP+) into Saccharomyces cerevisiae have been investigated in order to establish whether this compound can be used to monitor the membrane potential of his organism. Unlike dibenzyldimethylammonium, TPP+ is not translocated via the thiamine transport system, nor via another inducible translocation mechanism. On changing the experimental conditions the equilibrium potential of TPP+ varies according to expected changes of the membrane potential. TPP+ accumulation is higher in metabolizing cells than in non-metabolizing cells. In addition, decreasing the medium pH, addition of the proton conductor 2,4-dinitrophenol and addition of K+ all cause an apparent depolarization, whereas Ca2+ apparently hyperpolarizes the cell membrane. It is concluded that TPP+, if applied at low concentrations, can be used to measure the membrane potential of S. cerevisiae.

Uptake of the lipophilic cation dibenzyldimethylammonium into Saccharomyces cerevisiae. Interaction with the thiamine transport system.

The distribution ratio of the lipophilic cation dibenzyldimethylammonium between the cells of Saccharomyces cerevisiae and the medium appears to reflect changes in the membrane potential in a way that is qualitatively correct: the addition of a proton conductor or of an agent which blocks metabolism causes an apparent depolarization of the cell membrane; monovalent cations cause also a lowering of the equilibrium distribution, whereas the addition of divalent cations results in an increase of the partition ratio. However, uptake of dibenzyldimethylammonium and probably also of other liophilic cations proceeds via the thiamine transport system of the yeast. Dibenzyldimethylammonium transport is inducible, like thiamine transport. A kinetic analysis of the mutual interaction between thiamine and dibenzyldimethylammonium uptake shows that these compounds share a common transport system; moreover, dibenzyldimethylammonium uptake is inhibited complete by thiamine disulfide, a competitive inhibitor of thiamine transport and dibenzyldimethylammonium uptake in a thiamine-transport mutant is reduced considerably. It is concluded that one should be cautious when using lipophilic cations to measure the membrane potential of cells of S. cerevisiae.

New membrane formation and intercellular communication in the early Xenopus embryo. II. Theoretical analysis.

In conjunction with a previous analysis of the electrical networks formed by the Xenopus embryo during development from the 2-cell stage to the 16-cell stage, some theoretical aspects are investigated. A computer simulation method for the derivation of the specific membrane resistances from the measured equivalent resistances between different compartments of a multicellular biological system is described in detail. The interdependence of the equivalent junctional and nonjunctional resistances, and the possible role of the blastocoel in intercellular communication are analyzed. Assuming that no direct pathways exist between nonadjacent cells, the equivalent junctional and nonjunctional resistances, as well as the resulting coupling ratios are calculated for all pairs of cells in the 4-cell, 8-cell and 16-cell embryo. Previous studies on electrotonic coupling in the early Xenopus embryo are discussed.

New membrane formation and intercellular communication in the early Xenopus embryo.

The ionic permeability of the nonjunctional and newly formed junctional membranes was investigated in embryos of Xenopus laevis up to the onset of the fifth cleavage. Continuous measurements were made of the equivalent nonjunctional (R'o) and junctional resistances (R'i) in different pairs of adjacent cells separated by one of the four cleavage membranes formed in that period. The specific resistance of the nonjunctional membranes (ro) and of each cleavage membrane (ri) as a function of time were derived using a generally applicable computer simulation model. ro decreased from about 40 komega cm2 in the in the uncleaved egg to about 10 komega cm2 at the 16-cell stage, due to the insertion of a small fraction of the relatively permeable newly formed cleavage membranes into the outer surface. Superimposed on this overall decline, a transient decrease of ro was observed during each cycle, caused by a temporary partial separation of the peripheral parts of adjacent blastomeres. The changes in followed the same pattern. R'1 increased stepwise during each cleavage cycle. At the onset of each cleavage there were no significant differences in R'i as measured between different pairs of cells. After an initial phase of membrane formation ri of all cleavage membranes remained constant at about 400 omega cm2. In the states investigated the coupling ratio ranged from 0.8 to 1. It is argued that this close coupling could be the result of the highly impermeable outer surface even in the absence of specialized junctions in the intercellular membranes.