Hematopoietic development is a complex process that involves a large number of growth factors and cytokines. Many cytokines are known to act on more mature, lineage-restricted cells of the hematopoietic system. However, no specific factors have yet been identified that induce the expansion of the most primitive hematopoietic cells without also inducing differentiation. To search for such factors, we isolated novel cell lines from the yolk sac in order to identify genes important in early hematopoietic and endothelial development. This approach led to the discovery of B219, a sequence that is expressed in at least four isoforms in very primitive hematopoietic cell populations and which may represent a novel hemopoietin receptor. The recently published receptor for the obesity (ob) gene product (leptin) is an isoform of B219 with a nearly identical ligand binding domain. B219/obr is expressed in the yolk sac, early fetal liver, enriched hematopoietic stem cells and in a variety of lymphohematopoietic cell lines. B219/obr is also expressed at high levels in adult reproductive organs. B219/obr maps to human chromosome 1p32, a region syntenic with the recently reported location of obr on murine chromosome 4 (ref. 5).

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In an attempt to immortalize the gene products of single neurons, somatic cell hybrids were produced by fusion of embryonic rat dorsal root ganglion (DRG) neurons with mouse neuroblastoma cells. Embryonic day 13 rat DRGs were fused with mouse neuroblastoma cells deficient in hypoxanthine phosphoribosyltransferase (HPRT; IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8). The hybrid cells were selected in medium with 100 IM hypoxanthine/1 !LM aminopterin/12 ,LM thymidine to eliminate the neuroblastoma cells and with cis-hydroxyproline to retard fibroblast growth. Of the 17 lines derived, 4 manifested neuronal properties and were cloned. These lines retain both rat and mouse chromosomes and synthesize characteristic rat and mouse isoenzymes. Neuronal gangliosides, action potentials, and extensive neurite-like processes are exhibited by these hybrid cells, properties characteristic of DRG neurons but not of the neuroblastoma parent. Each line manifests a unique combination of action-potential properties and cell-surface markers, suggesting the selective expression of subsets of DRG neuronal genes.All of these neuronal properties are expressed constitutively, without the need for chemical induction or mitotic inhibition, and stably, without diminution after at least 5 months in culture. These lines may prove useful in the identification and isolation of gene products that characterize individual or small subsets of DRG neurons.

Chemoreceptors for oxygen reside within the carotid body, but it is not known which cells actually sense hypoxia and by what mechanisms they transduce this information into afferent signals in the carotid sinus nerve. We have developed systems for the growth of glomus cells of the carotid body in dissociated cell culture. Here we demonstrate that, as in vivo, these cells contain the putative neurotransmitters dopamine, serotonin, and norepinephrine. Oxygen tension regulates the rate of dopamine secretion from the glomus cells. Similar to chemically stimulated catecholamine secretion from other adrenergic cells this hypoxia-stimulated release requires extracellular calcium. These results are compatible with the suggestion that the glomus cells of the carotid body are chemoreceptor cells and that they signal hypoxia by regulated secretion of dopamine.Since the work of Heymans (1) it has been known that the hypoxic ventilatory drive originates primarily within the carotid body. Subsequent work has outlined the role of this sensory organ in control of breathing at sea level and at higher altitude (1, 2) and in the sensation of dyspnea (3). However, definition of the molecular basis of oxygen transduction has been hampered by the complexity of the carotid body. It contains several cell types, including endothelial, sustentacular, and glomus cells as well as the terminals of afferent and sympathetic nerves (4, 5), but it is not known which are actually capable of sensing the level of oxygen. The glomus cells have been attractive candidates to be chemoreceptor cells because they contain dopamine, a transmitter secreted from the carotid body during hypoxia, and because they form morphological synapses with afferent nerve terminals (5, 6). We have designed systems for the growth of glomus cells in culture in order to investigate the neurophysiology of these cells and their potential role in chemoreception (7). Here, we report evidence that carotid body glomus cells in culture synthesize and secrete putative neurotransmitters and manifest chemosensitivity in their control of dopamine secretion. MATERIALS AND METHODSDissociated newborn rat carotid body cells were cultured as described (7) in 24-well plates and were grown in a humidified atmosphere of Po2 151 mm Hg, Pco2 36 mm Hg in supplemented F-12 medium (pH 7.2). Nerve growth factor was not added to the medium. Enrichment for glomus cells could be achieved by the use of tyrosine-free medium (7), but successful elimination of other cells required several weeks of growth under these conditions. Since the very few cells in the carotid body was the limiting factor in the number of experiments that were possible and since there was a slow but noticeable decrement in catecholamine content in cultures >7-10 days old, we elected not to use this long-term enrichment procedure. Hypoxic conditions were identical except that the Po2 was diminished to 36 mm Hg. Since there are only a few thousand cells in the carotid body, each experiment necessitated the pooling of dissocia...

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