RT-PCR and pharmacological analysis of L-and T-type calcium channels in rat carotid body.

Mechanisms involved in carotid body (CB) chemoreceptor cells O(2)-sensing and responses are not fully understood. So far, it is known that hypoxia depolarizes chemoreceptor cells via O(2)-sensitive K(+)-channel inhibition; calcium influx via voltage-gated channels and neurotransmitter secretion follow. Presence of high voltage activated (HVA) calcium channels in rat CB chemoreceptor cells is well documented, but the presence of low voltage activated (LVH) or T-type calcium channels has not been reported to date. The fact that O(2)-sensitive PC12 cells express T-type channels and that they are inducible by chronic hypoxia (CH) lead us to hypothesize they could be present and play a role in the genesis of the hypoxic response in rat CB chemoreceptor cells. We have analyzed the expression of the three isoforms of T-type calcium channels (alpha1G, alpha1H and alpha1I) and the isoforms alpha1C and alpha1D of L-type calcium channels in rat CB by RT-PCR. We found that rat CB expresses alpha1G and alpha1C subunits. After chronic hypoxic treatment of adult rats (10 degrees O(2), 8 days), expression of alpha1G seems to be down-regulated whereas alpha1C expression is up-regulated. Functionally, it was found that the release of catecholamine induced by hypoxia and high external K({+}) from CB chemoreceptor cells was fully sensitive to L-type channel inhibition (nisoldipine, 2 microM), while specific inhibition of T-channels (mibefradil, 2 microM) inhibited exclusively hypoxia-induced release (50 degrees ). As a whole, present findings demonstrate the presence of T-type as well as L-type calcium channels in rat CB and suggest a selective participation of the T-type channels in the hypoxic activation of chemoreceptor cells.

Effects of bicarbonate buffer on acetylcholine-, adenosine 5'triphosphate-, and cyanide-induced responses in the cat petrosal ganglion in vitro.

Acetylcholine (ACh), adenosine 5'-triphosphate (ATP) and sodium cyanide (NaCN) activate petrosal ganglion (PG) neurons in vitro, and evoke ventilatory reflexes in situ, which are abolished after bilateral chemosensory denervation. Because in our previous experiments we superfused the isolated PG with solutions free of CO2/HCO3- buffer, we studied its effects on the PG responses evoked in vitro. PGs from adult cats were superfused at a constant pH, with HEPES-supplemented (5 mM) saline with or without CO2/HCO3- (5%/26.2 mM) buffer, and carotid (sinus) nerve frequency discharge (fCN) recorded. Increases in fCN evoked by ACh, ATP and NaCN in CO2- free saline were significantly reduced (P < 0.05, Wilcoxon test) when CO2/HCO3- was present in the superfusion medium. Thus, the presence of CO2/HCO3- buffer appears to reduce PG neurons sensitivity to ACh, ATP and NaCN, an effect that may underlie the lack of ventilatory reflexes after bilateral chemodenervation.

Effects of almitrine bismesylate in a microswine model of hypoxemic hypothermia.

We have developed an anesthetized microswine model of hypoxemic hypothermia and rewarming for testing prophylaxes and treatments. The respiratory stimulant almitrine bismesylate (ALM) was considered as a potential field expedient therapy for hypoxemic hypothermia. Preliminary experiments demonstrated that five consecutive 100 micrograms.kg-1 ALM intravenous (i.v.) doses given to normothermic microswine 3-4 min apart increased minute ventilation from an average of 3.4 L.min-1 to 4.5 L.min-1 (n = 2). However, when either a single i.v. ALM dose of 150 micrograms.kg-1 (n = 1) or three consecutive 100 micrograms.kg-1 i.v. doses given 15 min apart (n = 1) to hypoxemic hypothermic microswine with a mean esophageal temperature (Tes) = 28.8 degrees C, and a mean arterial O2 partial pressure (PaO2) = 49 mmHg, the hypoxemia was potentiated (mean PaO2 = 32 mmHg) and respiratory arrest ensued. Other experiments using continuous ALM i.v. infusion (1.0 microgram.kg-1.min-1) in hypoxemic hypothermic microswine (n = 6, Tes = 30.6 +/- 0.5, PaO2 = 55.4 +/- 12.9) did not demonstrate significant (p < or = 0.05) cardiorespiratory differences (ventilation, heart rate, blood pressure, blood gases) when compared to hypoxemic hypothermic controls (n = 6, Tes = 30.7 +/- 0.5, PaO2 = 53.3 +/- 13.6). These results suggest that high dose i.v. bolus administration of ALM is not indicated as a potential field expedient therapy for hypoxemic hypothermia, while further work is required to assess the potential efficacy of other continuous low dose i.v. infusion regimens.

Effects of a continuous infusion of dopamine on the ventilatory and carotid body responses to hypoxia in cats.

1. We investigated how a continuous infusion of dopamine (DA; 5 micrograms/kg per min), which is often used clinically, would affect the ventilation and carotid chemoreceptor neural activity in anaesthetized cats. 2. In anaesthetized, spontaneously breathing cats, tidal volume (VT) and respiratory frequency (f) were continuously monitored at five levels of inspired oxygen (PIO2 = 110, 130, 150, 170, 760 mmHg) during Da or saline infusion. VT and f were sampled for 1 min after 3 min exposure to each level of PIO2. Time control study was also performed. 3. DA infusion significantly lowered VT under both normoxia and hypoxia in seven of eight cats. Respiratory frequency was not affected by DA infusion. Depression of ventilation during post-hypoxic hyperoxia was augmented by DA infusion. Chemodenervation abolished the ventilatory response to hypoxia and DA did not further affect the ventilatory response to hypoxia. 4. In a second group of artificially ventilated cats, carotid chemoreceptor neural activity was recorded at five levels of arterial oxygen tension. DA infusion significantly depressed carotid chemoreceptor neural activity during normoxia and hypoxia in six of seven cats. 5. These findings suggest that changes in ventilation during low dosage of DA infusion closely correlate with carotid body neural output. A predominant effect of this dosage of DA (5 micrograms/kg per min) was depression in the ventilatory response to hypoxia due to an inhibition of carotid body neural output.

[Mexiletine, a drug with anti-arrhythmic action. Experimental research on the cardiovascular system]. / Il mexilitene, farmaco ad attività antiaritmica. Ricerche sperimentali sull'apparato cardiovascolare.

It was shown experimentally that mexilithene possesses anti-arrhythmic activity with slight depression of sino-carotid baroreceptorial and glomo-carotid and gangliar chemoreceptorial activity and catecholamine uptake. Hypotensive and bradycardizing effects were only noted when high per kg doses were used. The drug did not display vascular alpha- and beta-adrenolytic, anti-muscarinic and anti-histaminic activity.