Archives Internationales de Physiologie, de Biochimie et de Biophysique, 1991, 99, 323-329

323 Rqu le I5 octobre 1990.

ln-vivo and in-vitro studies on the effects of chronic dexamethasone treatment on cardiovascular responses to sympathetic stimulation BY

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MO-YIN CHAN, S. DAI, J. H. HE (') and C. W.OGLE (Department of Pharmacologv, Faculty of Medicine, University of Hong Kong, Hong Kong) (3 figures)

Rats treated with dexamethasone, 1.5 mg/kg S.C. weekly for 3 weeks exhibited significantly greater increases in mean arterial pressure than their controls, following either sympathetic nerve stimulation or noradrenaline administration. The atria from dexamethasone-treatedrats showed greater chronotropic activity in response to noradrenaline but not to field stimulation, whereas the force of contraction was significantlyless than that of the controls after either field or noradrenaline stimulation. Isolated rat taiI artery preparations from dexamethasone-treated rats were found to be twice more sensitive to noradrenaline than the controls. Prazosin antagonised the noradrenaline-induced pressor response to the same extent in control and dexamethasone-treatedrats. Dexamethasone treatment did not significantly increase the sensitivity to KCI or the angiotensin-potentiatedpressor response to noradrenaline. This study shows that dexamethasonetreatment increases postsynaptic sensitivity of the cardiovascular system to noradrenaline in rats.

Introduction It is well known that the glucocorticoidshave a profound effect on the cardiovascular system. Evidence has shown that the roIe of the glucocorticoids in the regulation of blood pressure is unrelated to their mineralocorticoid activity. Hypertension can be induced by steroids without association with marked disturbances in sodium metabolism or plasma volume (WHITWORTH et al., 1989). Recent work on glucocorticoid-induced hypertension has focused on changes in the levels of endogenous substances, such as renin and prostacyclin. The exact role of glucocorticoids in this regulatory aspect is, however, still controversial. Some investigators have shown that renin activity is unrelated to dexamethasone-induced hypertension (BURRISet ul., 1986), whereas others have demonstrated a definitive role for renin (YOSHIDA et al., 1988). Evidence is also equivocal with regard to the participation of prostacyclin and related eicosanoids (MIYAMORI et al., 1985; CODDE& BEJLIN, 1985; GRUNFELD et al., 1986). More recent findings suggest that enhanced vascular reactivity to vasopressin ( I m & MALIK, 1988) and decreased synthesis of arterial et al., 1988) may natriuretic peptide (ANP) (TONOLO be the causes underlying dexamethasone-induced hypertension. The interaction of glucocorticoids with noradrenaline has also been studied. In the isolated (') Dr. J. H.

rabbit aorta strip, addition of hydrocortisonehas been shown to potentiate smooth muscle contraction induced by noradrenaline (KALSNER, 1969). In man, orally administered hydrocortisone increases peripheral vascular resistance (SUDHIRet al., 1989). Thus, it is possible that adrenergic activities may be modified after prolonged exposure to large amounts of glucocorticoid. The present study examines this idea, using dexamethasonetreated rats and examining their cardiovascular responses to sympatheticnerve stimulation in viva The responses of isolated atria and artery preparations to electrical field stimulation and to noradrenaline were also studied.

Methods

Animals Male Sprague-Dawleyrats weighing 200-250 g were used. They were housed in an air-conditioned and humidity-controlled room (23 f 1°C and 60-70%, respectively), and allowed free access to tap water and a standard laboratory diet of rat chow (RalstonFurina, U.S.A.). The animals were injected S.C. with dexamethasone (Decadron-LA, M.S.D.) 1.5 mg/kg weekly for 3 weeks. Control rats were given equivalent volumes of 0.9% v/w NaCl (saline) by the same route. In order to verify the chronic effects of dexamethasone,the body weights

of the Department of Pharmacology, Tianjin Medical College, Tianjin, P. R. China was a Special Studies Fellow supported

by a China Medical Board Fellowship.

324 of dexamethasone-treatedanimals, as well as weights of their adrenal glands were recorded after completion of the experiment; these were compared with those of their saline-treated controls.

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Measurement of cardiovascularresponses to electrical sympathetic nerve or noradrenalinestimulation in vivo. Rats subjected to electrical sympathetic nerve stimulation were pithed under ether anaesthesia, using et al., 1970; a previously described method (GJLLESPJE LEUNGet al., 1986). The rats were pithed with a rod which comprised an external movable 6 cm long steel trocar and an internal 13 cm stainless steel electrode. The electrode was insulated by a layer of teflon, except for the terminal 4.5 cm. The exposed part of electrode was lodged at the level of T, to T12;the exact position was confirmed by post-mortem examination. The indifferent needle electrode was inserted into the subcutaneous tissues on the back of the animal, in parallel with the stimulating electrode. Ventilation was maintained artificially with room air (82 strokeshin, 1 ml/kg) using a respirator (Palmer, U.K.). Skeletal muscle contraction and parasympathetic excitation were minimised by pretreating the animals with gallamine triethiodide (Sigma) 5 mg/kg i.v. and atropine sulphate (E. Merck) 1 mg/kg i.p., 15 min before electrical stimulation. Electrical impulses (70 V, 0.5 msec pulse width and 1-8 Hz frequency) were provided by a square-wave stimulator (Palmer, C.V.P. model). Each stimulation lasted 5 s and was delivered at 5-min intervals. In experiments which evaluated the responses to injected noradrenaline, anaesthesia was induced by ether and maintained with chloralose (BDH) 100 mg/kg given i.v. through a cannulated jugular vein. Noradrenaline bitatrate (Sigma) 0.1-12.5 pg/kg was injected i.v. at 5-min intervals. In both experiments, the carotid arterial blood pressure was measured with a Statham P23ID pressure transducer. Integrated pulse rate was recorded by triggering the pressure wave using a biotachometer (Narco Bio-Systems). These variables were recorded on a physiograph (MK-IV, Narco Bio-Systems).

Rat isolated atrial preparations The method described by RAND et al. (1982) was used. Rats were stunned by a blow to the head and exsanguinated. Their hearts were removed and the atria dissected free. These were then mounted in an organ bath filled with Krebs-Henseleit solution containing atropine. The composition of the solution was (mM) : NaClll8, KC14.7, NaHCO, 25, MgS040.45, HK2P04 1.03, CaCl, 2.5, D-glucose 11.1 and atropine sulphate 0.005. The organ bath was gassed with a mixture of 5% C02 in O2 and maintained at 37°C. After an equilibration period of 20 min, the atria were stimulated either by electrical stimulation or by the addition of noradrenaline into the organ bath. The electrical field pulses were monophasic square waves of 15 V and of 0.5 ms duration. The atria were stimulated with pulses of variable frequency of 5 , 10, 15, 20, 30, 35 and 40 Hz. In the noradrenaline stimulation experiments,

MO-YIN CHAN, S. DAI, J. H. HE AND CL. W. OGLE

noradrenaline was added to the organ bath to achieve concentrations of to lo4 M. The force and rate of contraction were measured with a strain-gauge transducer (A-1082, E & M Instrument Co. Inc., Houston) and biotachometer (Narco Bio-systems), respectively.

Rat isolated tail artery preparations The technique described by XIAo & RAND (1989), with slight modifications, was employed. Rats were stunned by a blow to the head and exsanguinated. A segment of the tail artery was isolated and mounted vertically with the distal end upmost in an organ bath filled with modified Krebs solution gassed with mixture The artery was then perfusof 5% CO, and 95% 0,. ed, using a peristaltic pump (Minipuls 2, Gilson), with modified Krebs solution at a rate required to maintain a perfusion pressure of 40 mmHg. Perfusion pressure was measured by a Statham P23ID pressure transducer and recorded on a physiograph (Narco Bio-systems). The modified Krebs solution had the following composition (mM) :NaCl118, KC1 4.7, CaCl, 2.5, MgSO, 0.45, NaHCO, 25, KH,P04 1.03, D-glucose 11.1, disodium edetate 0.067. The preparations were maintained at a temperature of 37°C. After an equilibration period of 30 min, noradrenaline was added to the M. organ bath to achieve concentrations of lO-' to In inhibition studies, prazosin 10 1 1 was ~ added to the bath 5 min before the addition of noradrenaline. Various doses of angiotensin I1 and KC1 were added to the bath for stimulation studies. Drugs The following drugs were used : dexamethasone (Decadron-LA, M.S.D.), atropine sulphate (E. Merck), gallamine triethiodide (Sigma), angiotensin I1 (hypertensin, Ciba-Geigy),(-)-noradrenaline bitartrate (Sigma), and prazosin hydrochloride (Pfizer).

Statistical analysis Results were expressed as means f s.e.m. and analysed by Student's unpaired t-test. The frequency that produced 50% maximum response @F50) and pD, were obtained by analysis of the log frequency-response curves and log dose-response curves respectively. normal

pithed

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1. Basal blood pfesSUre and Puke rate in control (0) (n = 8) and dexamethaso*treated (H) (n= 8) pithed rats. The values plotted are the means f s.e.mean. FIG.

325

DEXAMETHASONE ENHANCES SYMPATHETIC RESPONSES

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FIG.2 . Effects of electrical sympathetic nerve stimulation on (a) mean blood pressure (MAP) and (b) pulse rate in control (0----0) (n = 8) and dexamethusone-treated (.----a) (n = 8) rats. The values plotted are the means f s.e.mean. *P

In-vivo and in-vitro studies on the effects of chronic dexamethasone treatment on cardiovascular responses to sympathetic stimulation.

Rats treated with dexamethasone, 1.5 mg/kg s.c. weekly for 3 weeks exhibited significantly greater increases in mean arterial pressure than their cont...
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