Relaxation induced by histamine is not endothelium dependent in tail arteries of L-NAME-treated rats

The role of autacoids and the autonomic nervous system in cardiovascular responses to radio-frequency energy heating

Among the potential effects of exposure to high levels of radio-frequency energy (RFE) (which includes microwaves), an increase in body temperature is the primary consequence. Release of autacoids and activity of the autonomic nervous system may influence (or be directly responsible for) some of the physiological changes that occur in conjunction with this hyperthermia. The main focus of this review is the interaction of autacoids and the autonomic nervous system with cardiovascular changes during heating. Differences between environmental and RFE-induced heating (such as rate of temperature change and degree of skin vs. core heating) may be important when considering these effects. Antihistamines exhibited no beneficial effect on circulatory collapse during RFE-induced heating. The serotonergic blocker methysergide decreased survival time in rats during terminal RFE exposure, despite no effects on heart rate (HR) or blood pressure. Although blockade of platelet-activating factor resulted in lower HR before RFE exposure, there was a lack of effect on the subsequent increase in HR during heating. Nitric oxide did not contribute to the hypotension that occurs due to rapid heating by RFE exposure. There have been either no or very limited studies of effects of prostaglandins, bradykinin, or angiotensin on RFE-induced heating responses. beta-Adrenoceptor antagonism with propranolol resulted in significantly decreased survival times and lower final colonic temperatures during RFE exposure. A lack of effects of nadolol on survival time and temperature, coupled with its poor ability to traverse the blood-brain barrier, suggests that central beta-adrenergic stimulation rather than peripheral stimulation may alter thermoregulation. Effects of the autonomic nervous system (as studied by adrenoceptor blockade) on potassium changes during heating have not been fully investigated. Such changes could be important in animals' responses to RFE and other modalities of heating, and should be studied in future.

Disruption of microtubular network attenuates histamine-induced dilation in rat mesenteric vessels

Cytoplasmic microtubules are important in many cellular homeostatic processes in the cell. They regulate cell shape and movement as well as serving as a network by which vesicles and membrane-bound organelles can travel. Lately, there have been many studies demonstrating that microtubules are involved in regulation of intracellular signaling and, therefore, affect vascular reactivity. In this study, we tested the hypothesis that microtubule disruption attenuates agonist-induced endothelium-dependent vasodilation. Isolated mesenteric arterial bed from normotensive rats was preconstricted with phenylephrine, and dose-response curves for histamine, acetylcholine (ACh), sodium nitroprusside (SNP), and pinacidil were performed before and after incubation with nocodazole or colchicine. Treatment of the vascular beds with nocodazole or colchicine significantly attenuated histamine relaxation but did not change the ACh-, SNP-, or pinacidil-induced vasorelaxation. Nocodazole did not cause an additional attenuation of the histamine-mediated dilation in mesenteric vessels in the presence of Nω-nitro-l-arginine methyl ester, high extracellular K+, or K+channel blockers. These data suggest that disruption of microtubules affects an essential endothelial component of histamine-mediated vasodilation in the mesenteric arterial bed. The mechanism(s) involved in this effect might be related to an impairment of endothelial NO synthesis, which might not be as important for the ACh as for the histamine vasodilator response in rat mesenteric vessels. These results demonstrate the importance of the microtubular system for endothelium-dependent NO-mediated smooth muscle relaxation.

Functional Study of the [Ca2+]i Signaling Pathway in Aortas of L-NAME-Hypertensive Rats

A variety of mechanisms has been proposed to suggest that nitric oxide participates in the regulation of smooth muscle free [Ca(2+)](c) (the primary determinant of contractile tone), including inhibition of Ca(2+) influx across the plasma membrane and inhibition of intracellular Ca(2+) release. In view of such considerations, the aim of this study was to investigate the possible alterations in contractile responses induced by drugs that mobilize Ca(2+) from different sources in aortae from N(G)-nitro-L-arginine methyl ester (L-NAME) hypertensive rats (LHR). Treatment with L-NAME did not alter the contractile response induced by phenylephrine; however, indomethacin increased the contraction to phenylephrine only in LHR aortae (1.36 +/- 0.08 g, n = 6, vs. 1.97 +/- 0.09 g, n = 7). Both phenylephrine and caffeine evoked rapid and phasic contractions in intact or denuded aortic rings in Ca(2+)-free solution containing EGTA. Phenylephrine-elicited phasic contractions were lower in normotensive rats (NR; 0.41 +/- 0.05 g, n = 9) than in LHR (0.57 +/- 0.06 g, n = 6) and were increased by endothelium removal only in the NR group (0.64 +/- 0.05 g, n = 6). Conversely, neither with treatment with L-NAME nor endothelium removal altered the phasic contractile responses induced by caffeine. The Ca(2+) influx stimulated with phenylephrine was greater in NR (1.95 +/- 0.08 g; pD(2) 6.06 +/- 0.69; n = 8) than in the LHR denuded aorta (1.63 +/- 0.11 g; pD(2) 3.52 +/- 0.06; n = 6). Similarly, contractions stimulated with phorbol ester in denuded arteries were greater in NR (1.76 +/- 0.08 g, n = 7) than in LHR (1.11 +/- 0.11 g, n = 7). In the same manner, indomethacin failed to alter the contraction stimulated with phorbol ester in NR arteries (2.01 +/- 0.21 g, n = 7), although it completely blocked the inhibitory effect of chronic treatment with L-NAME on this contractile response (1.94 +/- 0.24 g; n = 9). Indomethacin did not change the contractile responses stimulated by increasing concentrations of extracellular Ca(2+) in either NR aortas (1.44 +/- 0.26 g; pD(2) 4.74 +/- 0.79; n = 6) or LHR aorta (1.99 +/- 0.19 g; pD(2) 4.10 +/- 0.47; n = 8). However, in the presence of indomethacin, the Ca(2+) influx was similar in NR and LHR aortae. Taken together, these results suggest that, in this model of hypertension, the increase in agonist-induced release of Ca(2+) from intracellular stores may be partly compensated by inhibition of Ca(2+) influx and that this effect is due to the increased production of the relaxant prostanoid in vascular smooth muscle cells.

Effects of histamine receptor blockade on cardiovascular changes induced by 35 GHz radio frequency radiation heating

1. The role of histamine in heat-induced cardiovascular changes is uncertain. The purpose of this study was to examine effects of histamine H-1- and H-2-antagonism on heart rate, mean arterial blood pressure (MAP), localized body temperature changes, survival times, and lethal body temperatures that occur during the exposure of anaesthetized rats to 35 GHz radio frequency radiation (RFR). 2. Forty-eight ketamine-anaesthetized Sprague-Dawley rats were exposed, in several different treatment groups (n = 8 in each), to 35 GHz RFR at a level that resulted in significant body heating and subsequent death. During irradiation, a continuous increase in heart rate and a biphasic response in blood pressure (initial increase followed by a decrease) were observed in all groups of animals. 3. An H-1-antagonist, diphenhydramine (1 mg kg(-1) body wt) and an H-2-antagonist, cimetidine (5 mg kg(-1)), administered after sustained RFR exposure, failed to reverse the RFR-induced hypotension. High doses of the drugs (5 and 10 mg kg(-1), respectively) also did not alter the response. Post-RFR survival time was significantly decreased in the high-dose drug-treated group, compared with vehicle-treated (0.9% NaCl, 50% ethanol and 50% D5W) controls. 4. In experiments in which the two drugs were administered prior to RFR exposure, MAP in animals receiving high-dose antihistamines was significantly depressed compared with that of vehicle-treated animals during the first 35 min of RFR exposure. Antihistamine pretreatment, however, did not alter the total RFR exposure time required for death to occur. 5. In summary, pharmacological blockade of H-1 and H-2 receptors is not beneficial in anaesthetized rats made hypotensive by RFR exposure. This indicates that activation of H-1 and H-2 receptors by histamine does not occur to any significant extent and does not mediate the hypotensive response developed in this model of hyperthermia.