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The sensing of blood gas tensions and/or pH is an evolutionarily conserved, homeostatic mechanism, observable in almost all species studied from invertebrates to man. In vertebrates, a shift from the peripheral [O.sub.2]-oriented sensing in fish, to the central C[O.sub.2]/pH sensing in most tetrapods reflects the specific behavioral requirements of these two groups whereby, in teleost fish, a highly [O.sub.2]-oriented control of breathing matches the ever-changing and low oxygen levels in water, whilst the transition to air-breathing increased the importance of acid-base regulation and [O.sub.2]-related drive, although retained, became relatively less important. The South American lungfish and tetrapods are probably sister groups, a conclusion backed up by many similar features of respiratory control. For example, the relative roles of peripheral and central chemoreceptors are present both in the lungfish and in land vertebrates. In both groups, the central C[O.sub.2]/pH receptors dominate the ventilatory response to hypercarbia (60-80%), while the peripheral C[O.sub.2]/pH receptors account for 20-30%. Some basic components of respiratory control have changed little during evolution. This review presents studies that reflect the current trends in the field of chemoreceptor function, and several laboratories are involved. An exhaustive review on the previous literature, however, is beyond the intended scope of the article. Rather, we present examples of current trends in respiratory function in vertebrates, ranging from fish to humans, and focus on both [O.sub.2] sensing and C[O.sub.2] sensing. As well, we consider the impact of chronic levels of hypoxia--a physiological condition in fish and in land vertebrates resident at high elevations or suffering from one of the many cardiorespiratory disease states that predispose an animal to impaired ventilation or cardiac output. This provides a basis for a comparative physiology that is informative about the evolution of respiratory functions in vertebrates and about human disease. Currently, most detail is known for mammals, for which molecular biology and respiratory physiology have combined in the discovery of the mechanisms underlying the responses of respiratory chemoreceptors. Our review includes new data on nonmammalian vertebrates, which stresses that some chemoreceptor sites are of ancient origin.

We have studied the sensitivity of soft error rate (SER) to an external magnetic field in longitudinal hard disk drives. We found that during writing SER is generally quadratic with field amplitude and symmetric with field direction. The application of an external field during writing also shifts the center of the track. In contrast, during reading SER is predominantly linear with field amplitude and antisymmetric with field direction. There is no shift in the track center when the field is applied during reading up to 100 Oe. We explain the difference in the track center shift between writing and reading in terms of the effect of the stray field on the servo positioning system. Index Terms--Longitudinal recording, perpendicular recording, servo system, stray field.

Summary: Physical characteristics of water (O2 solubility and capacitance) dictate that cardiovascular and ventilatory performance be controlled primarily by the need for oxygen uptake rather than carbon dioxide excretion, making O2 receptors more important in fish than in terrestrial vertebrates. An understanding of the anatomy and physiology of mechanoreception and O2 chemoreception in fishes is important, because water breathing is the primitive template upon which the forces of evolution have modified into the various cardioventilatory modalities we see in extant terrestrial species. Key to these changes are the O2-sensitive chemoreceptors and mechanoreceptors, their mechanisms and central pathways. [Copyright &y& Elsevier]

Abstract: Little is known about the cells or mechanisms of O2 chemoreception in vertebrates other than mammals. The purpose of this study, therefore, was to identify O2-sensitive chemoreceptors in a fish. Putative O2-sensitive chemoreceptors were dissociated from the gills of channel catfish, Ictalurus punctatus, and cultured. A population of cells was identified with morphology and a histochemical profile similar to mammalian carotid body Type I (glomus) cells and pulmonary neuroepithelial cells. These cells stain with neutral red and appear to be the branchial neuroepithelial cells. Immunocytochemical staining showed that these cells contain neuron-specific enolase (NSE), tyrosine hydroxylase (TH) and 5-hydroxytryptamine (5HT). Patch-clamp experiments showed that these cells have a O2-sensitive, voltage-dependent outward K+ current like mammalian O2 sensors. Two kinds of electrophysiological responses to hypoxia (PO2 < 10 Torr) were observed. Some cells showed inhibition of outward current in response to hypoxia, whereas other cells showed potentiation. Neurochemical content and electrophysiological responses to hypoxia indicate that these cells are piscine O2-sensitive chemoreceptors. [Copyright &y& Elsevier]

Abstract: The effects of cobalt chloride on heart rate, blood pressure, ventilatory frequency and opercular pressure amplitude in channel catfish, Ictalurus punctatus were measured to evaluate the potential of cobalt as a histochemical probe to study mechanisms of oxygen chemoreception, as well as assess the general effects of cobalt on the cardioventilatory physiology of fishes. Cobalt, like cyanide, has been previously used to stimulate oxygen chemoreceptors and hypoxic reflexes in mammals but there is little information on the cardioventilatory effects of cobalt on fish. Catfish were exposed to increasing concentrations (1–20 mg/kg) of cobalt in the water (external) or injections into the dorsal aorta (internal) and the cardioventilatory effects recorded. Mean arterial pressure showed a significant, dose-dependent increase in response to cobalt injections. Heart rate increased slowly, but significantly after cobalt injections but the magnitude of change was not dose-dependent. There was a small increase in ventilatory rate but no effect on amplitude. External cobalt had similar effects but the responses were weaker. Although cobalt stimulated some cardioventilatory reflexes the pattern and magnitude of the responses were noticeably different from those of cyanide and hypoxia. The results suggest that the cardioventilatory reflexes stimulated by cobalt were not mediated by O2-sensitive chemoreceptors and that cobalt is not an effective O2 receptor stimulant in fishes. [Copyright &y& Elsevier]

Presents a study which examined the effects of exhausting activity in spotted gar, Lepisosteus oculatus, to determine the role of the air-breathing organ in supporting active metabolism and recovery. Monitoring of blood gas, acid-base and cardiovascular variables; Similarity of metabolic effects of exercise to those in unimodal water breathing fish; Methodology used to conduct study; Results of study.

Abstract: Currently, we have little information concerning the involvement of β3-adrenergic receptors (AR) in cardiovascular regulation in fishes. The goal of this study was to investigate the effect of β3-AR ligands on in vivo cardiovascular function in larval and adult rainbow trout (Oncorhynchus mykiss). In adult fish, injection of BRL37344 (β3-AR agonist) resulted in an increase in heart rate (f H) (~31%) while stroke volume (Sv) was reduced (−25.9%). Injection of SR59230A (β3-AR antagonist) and propranolol (β1/β2-AR antagonist) resulted in increases in dorsal aorta blood pressure (PDA) with differing effect on cardiac variables (f H and Sv). To confirm specificity of the results, BRL37344 was injected following sequential injections of phentolamine (α1-AR antagonist), atropine (muscarinic antagonist), propranolol and SR59230A. While phentolamine had no effect on BRL37344, atropine completely abolished the influence of BRL37344 on f H, Sv and cardiac output (Q). In larval trout, BRL37344 (10 and 100μM) induced a significant concentration-dependent increase in f H while SR59230A (1 and 10μM) and propranolol (1 and 10μM) separately caused a significant concentration-dependent decrease. These data suggest that β3-ARs have an important role in regulation of cardiovascular function, and provide evidence for a potential interaction between muscarinic and adrenergic receptors in rainbow trout. [Copyright &y& Elsevier]