The biophysical and pharmacological properties of an oxygen‐sensitive background K+ current in rat carotid body type‐I cells were investigated and compared with those of recently cloned two pore domain K+ channels. Under symmetrical K+ conditions the oxygen‐sensitive whole cell K+ current had a linear dependence on voltage indicating a lack of intrinsic voltage sensitivity. Single channel recordings identified a K+ channel, open at resting membrane potentials, that was inhibited by hypoxia. This channel had a single channel conductance of 14 pS, flickery kinetics and showed little voltage sensitivity except at extreme positive potentials. Oxygen‐sensitive current was inhibited by 10 mM barium (57 % inhibition), 200 μM zinc (53 % inhibition), 200 μM bupivacaine (55 % inhibition) and 1 mM quinidine (105 % inhibition). The general anaesthetic halothane (1.5 %) increased the oxygen‐sensitive K+ current (by 176 %). Halothane (3 mM) also stimulated single channel activity in inside‐out patches (by 240 %). Chloroform had no effect on background K+ channel activity. Acidosis (pH 6.4) inhibited the oxygen‐sensitive background K+ current (by 56 %) and depolarised type‐I cells. The pharmacological and biophysical properties of the background K+ channel are, therefore, analogous to those of the cloned channel TASK‐1. Using in situ hybridisation TASK‐1 mRNA was found to be expressed in type‐I cells. We conclude that the oxygen‐ and acid‐sensitive background K+ channel of carotid body type‐I cells is likely to be an endogenous TASK‐1‐like channel.

The single channel properties of TASK-like oxygen-sensitive potassium channels were studied in rat carotid body type 1 cells. We observed channels with rapid bursting kinetics, active at resting membrane potentials. These channels were highly potassium selective with a slope conductance of 14-16 pS, values similar to those reported for TASK-1. In the absence of extracellular divalent cations, however, single channel conductance increased to 28 pS in a manner similar to that reported for TASK-3. After patch excision, channel activity ran down rapidly. Channel activity in inside-out patches was markedly increased by 2 and 5 mM ATP and by 2 mM ADP but not by 100 microM ADP or 1 mM AMP. In cell-attached patches, both cyanide and 2,4-dinitrophenol strongly inhibited channel activity. We conclude that 1) whilst the properties of this channel are consistent with it being a TASK-like potassium channel they do not precisely conform to those of either TASK-1 or TASK-3, 2) channel activity is highly dependent on cytosolic factors including ATP, and 3) changes in energy metabolism may play a role in regulating the activity of these background K+ channels.

Bone adapts to its environment by a process in which osteoblasts and osteocytes sense applied mechanical strain. One possible pathway for the detection of strain involves mechanosensitive channels and we sought to determine their sensitivity to membrane strain and tension. We used a combination of experimental and computational modeling techniques to gain new insights into cell mechanics and the regulation of mechanosensitive channels. Using patch-clamp electrophysiology combined with video microscopy, we recorded simultaneously the evolution of membrane extensions into the micropipette, applied pressure, and membrane currents. Nonselective mechanosensitive cation channels with a conductance of 15 pS were observed. Bleb aspiration into the micropipette was simulated using finite element models incorporating the cytoplasm, the actin cortex, the plasma membrane, cellular stiffening in response to strain, and adhesion between the membrane and the micropipette. Using this model, we examine the relative importance of the different cellular components in resisting suction into the pipette and estimate the membrane strains and tensions needed to open mechanosensitive channels. Radial membrane strains of 800% and tensions of 5 10(-4) N.m(-1) were needed to open 50% of mechanosensitive channels. We discuss the relevance of these results in the understanding of cellular reactions to mechanical strain and bone physiology.

SUMMARY1. We have studied the reflex respiratory responses to two-breath alternations of fractional inspired oxygen (FI,°2) 3. The degree of alternation in tidal volume, inspiratory time, expiratory time, frequency, drive, timing and instantaneous ventilation components of the respiratory response was compared during control and test runs.4. There was little response to control runs in either group at any post-natal age.5. In normoxic kittens we found no significant reflex response in any respiratory variable to test runs before day 4. However significant alternation was found in expiratory time, frequency and ventilation at days 4-8 and in tidal volume at days 9-14.6. In chronically hypoxic kittens there were no significant differences between control and test runs at any of the ages studied.7. In the normoxic group increases in the response with post-natal age probably reflect post-natal increases in the sensitivity of the peripheral chemoreceptors. The lack of development in the chronically hypoxic group may indicate abnormal function or delayed maturation of the peripheral chemoreceptor sensitivity to hypoxia.8. The results suggest that the method can be used to detect developmental and pathological changes in the arterial chemoreflex.