Activation of heme oxygenase and consequent carbon monoxide formation inhibits the release of arginine vasopressin from rat hypothalamic explants. Molecular linkage between heme catabolism and neuroendocrine function

Abstract

Heme oxygenase (HO)-catalyzed degradation of cellular heme moieties generates biliverdin and equimolar amounts of carbon monoxide (CO), which has been implicated as a possible modulator of neural function. Technical difficulties preclude direct measurements of CO within intact nervous tissues; hence, alternative procedures are needed to monitor the formation and possible biologic functions of this gas. In the present study rat hypothalamic explants were found to generate 114 ± 5 or 127 ± 11 pmol biliverdin/hypothalamus/1h ( n = 3) upon incubation with 1 or 10 mM hemin, respectively. Ten micromolar zinc-protoporphyrin IX (Zn-PP-IX), a known inhibitor of HO, significantly decreased the degradation of 10 µM hemin from 127 ± 11 to 26 ± 11 pmol biliverdin/hypothalamus/1h ( n = 3; P < 0.01). Biliverdin was the principal product of HO-dependent heme degradation, as its possible conversion into bilirubin was precluded by hemin-dependent inhibition of biliverdin reductase. Basal or hemin-supplemented hypothalamic incubations were also shown to generate sizable amounts of propentdyopents (PDPs), reflecting HO-independent degradation pathways which do not liberate CO and cannot be inhibited by Zn-PP-IX. Plotting the ratio of biliverdin to PDPs thus provided an index of the efficiency with which hemin was degraded through biochemical pathways involving CO. Under the experimental conditions of our study, the biliverdin/PDPs ratio varied from 0 to 32 or 15%, depending on the absence or presence of 1 or 10 µM hemin respectively: this suggested that the formation of CO was most efficient at 1 µM hemin. Under these defined conditions, 1 µM hemin was also found to inhibit the release of arginine vasopressin (AVP) evoked by depolarizing solutions of KCl. A series of experiments showed that the effect of hemin was counteracted by Zn-PP-IX, and also by tin-mesoporphyrin IX, which is even more selective in inhibiting HO; it was also attenuated in the presence of the gaseous scavenger ferrous hemoglobin. Furthermore, the inhibition of AVP release could be reproduced by omitting hemin and by incubating hypothalami under CO, whereas treatment with biliverdin had no effect. This suggested that the release of AVP was suppressed by HO degradation of hemin, yielding CO as a modulator of hypothalamic function. These observations may be relevant to diseases characterized by inappropriate secretion of AVP and enzymatic disturbances affecting the synthesis of heme and the formation of CO through the HO pathway (e.g., acute intermittent porphyria or lead intoxication).