Differential effects of the calcium antagonist, nisoldipine, versus the arterial vasodilator, minoxidil, on ventricular anatomy, intravascular volume, and sympathetic activity in rats

Opposite effects on cardiac volume load or sympathetic activity by calcium-antagonists versus classical arterial vasodilators may be responsible for their different effects on ventricular anatomy, [i.e., left ventricular (LV) and right ventricular (RV) weights and LV internal diameter and wall thickness.] Therefore, we evaluated the time course of changes in intravascular volume and ventricular anatomy in unanesthetized normotensive rats, following treatment for 1, 2, 14, 35, or 70 days with minoxidil (120 mg/L drinking water) or nisoldipine (0.3 and 1.0 mg/g food). Tissue-specific sympathetic activity was assessed by the norepinephrine turnover rate (NE TR) after 14 and 35 days of treatment. Minoxidil produced RV hypertrophy and eccentric LV hypertrophy (i.e., increased LV diameter with unchanged wall thickness) and increased intravascular volume. Nisoldipine did not alter LV anatomy, but the high dose produced a small, significant increase in RV weight, associated with a tendency (p less than 0.1) to increase blood volume. Minoxidil increased NE TR in the LV (after 14 and 35 days), in the RV (after 14 days), as well as in the superior mesenteric artery (after 14 and 35 days), but not in the kidney. Nisoldipine decreased cardiac NE TR, but did not affect NE TR in the other tissues, suggesting a central effect of nisoldipine. We conclude that an increase in blood volume caused by arterial vasodilators may contribute to cardiac volume overload resulting in cardiac hypertrophy. These volume and cardiac changes are largely absent during treatment with calcium-antagonists. Changes in cardiac sympathetic activity possibly modulate (i.e., potentiate or blunt) the extent of cardiac hypertrophy induced by cardiac overload.

Effect of minoxidil on platelet function and the synthesis of prostaglandins in platelets

At the 12.5 micrograms level, minoxidil prevents the irreversible aggregation of platelets by 2 x 10(-6) mol/L adenosine diphosphate (ADP). Levels of minoxidil greater than 12.5 micrograms cause a reversal of primary aggregation by 2 x 10(-6) mol/L ADP. Aggregation of platelets in response to 125 micrograms of arachidonic acid is measurably reduced by 12.5 micrograms of minoxidil and totally suppressed by 30 micrograms. Concurrent with the inhibition of platelet aggregation, increasing concentrations of minoxidil cause a gradual reduction in the synthesis of prostaglandin E2 (PGE2) and thromboxane B2 (TxB2). In the presence of 100 micrograms of minoxidil, PGE2 is reduced from a control value of 87.7 +/- 2.2 pg/ml to 23.9 +/- 3.2 pg/ml. At this level of minoxidil, TxB2 drops from 105 +/- 3.3 ng/ml to 10.5 +/- 2.6 ng/ml. The effect of minoxidil on platelet aggregation is not associated with increased cyclic adenosine monophosphate synthesis. All data support the conclusion that minoxidil functions (in platelet metabolism) primarily as a cyclooxygenase inhibitor.

Effects of putative activators of K+ channels in mouse pancreatic beta-cells

1 The vasodilator and antihypertensive properties of pinacidil, cromakalim (BRL 34915), nicorandil and minoxidil sulphate may be due, at least in part, to their ability to open K+ channels in vascular smooth muscles. In this study, mouse pancreatic islets were used to determine whether these drugs affect insulin release by acting on K+ channels of beta-cells. Their effects were compared to those of diazoxide. 2 Diazoxide caused a dose-dependent inhibition of insulin release by islets incubated with 15 mM glucose (93% at 100 microM). Pinacidil inhibited release by 36 and 72% at 100 and 500 microM, respectively. Cromakalim and nicorandil were less effective (35 and 25% inhibition at 500 microM). Minoxidil sulphate increased insulin release at 500 microM. 3 In the presence of 7 mM glucose and in the absence of Ca2+ (to avoid activation of Ca2+-dependent K+ channels), 86Rb efflux from islet cells was increased by 100-500 microM pinacidil and 500 microM nicorandil, which were, however, less potent than diazoxide. Cromakalim was ineffective, whereas 500 microM minoxidil sulphate decreased the efflux rate. In the absence of glucose and presence of Ca2+, 500 microM cromakalim and minoxidil sulphate inhibited 86Rb efflux. 4 Like diazoxide, pinacidil (500 microM) abolished glucose-induced electrical activity in beta-cells and hyperpolarized the membrane. 5 ATP-sensitive K+ currents were studied in single beta-cells by the whole cell patch-clamp technique. Pinacidil increased the current less than did diazoxide. In contrast, cromakalim and minoxidil sulphate decreased K+-currents whilst nicorandil was without effect. 6. It is concluded that pinacidil, like diazoxide, inhibits insulin release from beta l-cells by opening ATP-sensitive K+ channels, whereas the smaller inhibitory effects of cromakalim and nicorandil may involve actions other than on K+ channels in these cells. Minoxidil sulphate potentiates glucose-induced insulin release, probably by inhibiting ATP-sensitive K+ channels. However, all these effects of the vasodilators are only seen at high concentrations and are thus unlikely to occur in vivo.

Electrophysiological mechanisms of minoxidil sulfate-induced vasodilation of rabbit portal vein

The electrophysiological and mechanical properties of the vasodilator minoxidil sulfate (MNXS) were examined in isolated smooth muscle cells and strips from rabbit portal vein. At micromolar concentrations, MNXS inhibited norepinephrine (0.1-1.0 microM)-induced contractions in isolated muscle strips. In isolated cells, norepinephrine caused a dose-dependent depolarization of the resting membrane potential, which was significantly attenuated by MNXS (5 microM); MNXS alone caused a hyperpolarization of the membrane potential. This hyperpolarization was insensitive to Na+-K+ pump blockade by ouabain, but was inhibited by the K+ channel antagonist, tetraethylammonium (20 mM). In voltage-clamp experiments, a resting (background) conductance associated with the resting membrane potential was identified. This conductance, which previously has been shown to be reduced by Ba2+ as well as tetraethylammonium, was increased by MNXS (2 microM). In additional experiments, whole-cell L-type Ca2+ currents were inhibited by micromolar concentrations of MNXS. These experiments show that concentrations of MNXS that inhibit norepinephrine-induced contractions promote K+ conductance and inhibit Ca2+ entry through voltage-dependent Ca2+ channels in vascular smooth muscle cells. These electrophysiological effects of MNXS may be responsible for the vasorelaxant effects of the drug observed in vitro and in vivo.

The effect of minoxidil on endothelial cells in vitro

Minoxidil - a therapeutic agent used in the control of hypertension and which has been implicated in modulation of hair follicle activity - has been shown to influence certain aspects of the physiology of cultured endothelial cells. It suspends cell proliferation in a reversible manner, appears to influence cell morphology in a dose dependent way, does not significantly alter (qualitatively) protein synthesis and does not inhibit cell migration. These results demonstrate similarities between endothelial cells and keratinocytes and dermal fibroblasts and differences between endothelial cells and dermal papilla cells, follicular root sheath fibroblasts and neutrophils in their response to this agent.

Pseudo renal artery stenosis (PRAS) syndrome

During the course of a long-term, prospective, randomized study in 77 hypertensive nephrosclerosis patients, five patients developed evidence suggestive of renal artery stenosis. However, arteriography demonstrated patent renal arteries. The evidence suggestive of renal artery stenosis was: (1) converting-enzyme inhibitor (CEI)-induced renal dysfunction including marked and reversible increases in serum creatinine and urea concentrations, (2) minoxidil-induced hyperreninemia despite beta-adrenoceptor blockade and volume expansion, and (3) minoxidil-induced salt and water retention with diuretic resistant edema. Thus, the renal dysfunction induced by CEI in these patients with patent renal arteries is similar to the alterations occurring in patients having bilateral renal artery stenosis. The diuretic resistant edema and the beta-adrenoceptor blocker resistant high renin release are also functional alterations of renal artery stenosis. We suspect that the long-standing and usually severe hypertension in these patients has caused sufficient arteriolar hypertrophy or sclerosis to interfere with renal blood flow and to induce these functional lesions of renal artery stenosis. With widespread use of the new CEI agents in patients with renal disease, this syndrome suggestive of renal artery stenosis may be encountered in as many as 10% of hypertensive nephrosclerosis patients during long-term treatment with converting-enzyme inhibitors.

Simultaneous two-probe laser Doppler velocimetric assessment of topically applied drugs in rats

A simple, noninvasive method for the determination of cutaneous blood flow in anesthetized rats is presented. Simultaneous two-probe laser doppler velocimetry is shown to be a useful preclinical tool for the assessment of topically applied drugs.

Minoxidil stimulates mouse vibrissae follicles in organ culture

Minoxidil, a potent vasodilator, stimulates the growth of terminal hair from vellus or miniaturized follicles in balding scalp. To study minoxidil's action on isolated follicles we developed and validated an organ culture system using mouse whisker follicles. Control follicles cultured without minoxidil showed macroscopic changes including kinking of the hair shafts and bending of the follicles. Necrosis was evident in the differentiating epithelial elements forming the cuticle, cortex, and inner root sheath. These abnormalities were eliminated or greatly reduced in minoxidil-treated follicles. The morphology of these follicles was consistent with the production of new hair during culture. Direct measurement demonstrated that minoxidil-treated follicles grew significantly longer than control follicles during the 3-d culture. Minoxidil increased the incorporation of radiolabeled cysteine and glycine in follicles compared with control treatment. Doses of minoxidil up to 1 mM caused increased cysteine incorporation, while higher doses were inhibitory. Experiments with labeled thymidine indicated that minoxidil induced proliferation of hair epithelial cells near the base of the follicle. Autoradiography also showed that cysteine accumulated in the keratogenous zone above the dermal papilla. These studies demonstrate that organ cultured follicles are suitable for determining minoxidil's mechanism of action and may be useful for studying other aspects of hair biology. The results also show that minoxidil's effect on hair follicles is direct. This suggests that minoxidil's action in vivo includes more than just increasing blood flow to hair follicles.

Arterial vasodilators, cardiac volume load, and cardiac hypertrophy in normotensive rats

To assess a possible involvement of cardiac volume overload in the development of cardiac hypertrophy during chronic arterial vasodilator treatment, changes in indexes of cardiac volume load in relation to changes in cardiac anatomy were evaluated during treatment of normotensive rats with 120 mg/l hydralazine or 120 mg/l minoxidil, with drinking water. Long-term treatment with hydralazine, but not minoxidil, caused small decreases in systolic blood pressure; neither vasodilator affected heart rate with chronic treatment. Arterial vasodilator treatment for 2 wk or more resulted in increases in plasma and blood volumes by 10-20%. Both arterial vasodilators increased right atrial pressure and left ventricular end-diastolic pressure in the initial weeks of treatment. Only the minoxidil group showed a persistent increase in right atrial pressure throughout the treatment period. These hemodynamic changes were associated with increases in left ventricular (LV) internal diameter and right ventricular (RV) weight, and with minoxidil these changes were also associated with increased LV weight. LV wall thickness did not increase. Cardiac volume overload therefore indeed occurs during treatment with arterial vasodilators and may contribute to their effects on cardiac anatomy (i.e., development of RV hypertrophy and, in the case of minoxidil, also, eccentric LV hypertrophy), which are consistent with cardiac volume overload.

Decreased thermal denaturation temperature of osteogenesis imperfecta mutant collagen is independent of post-translational overmodifications of lysine and hydroxylysine

Fibroblasts from many patients with osteogenesis imperfecta (OI) synthesize and secrete Type I collagen which is both overmodified and exhibits a decreased thermal denaturation temperature. We have examined the relationship between overmodification and decreased melting temperature in several favorable OI mutants by selectively inhibiting lysyl hydroxylase activity with the drug Minoxidil and comparing the melting profiles of the resultant undermodified collagen with untreated control. Minoxidil treatment causes an appreciable decrease in hydroxylysine with compensatory increases in lysine content, and the delayed sodium dodecyl sulfate-polyacrylamide gel electrophoretic mobility of the overmodified collagen chains becomes normal. However, the decreased melting temperature was unchanged from untreated OI control. When unhydroxylated collagen produced by normal control and OI fibroblasts incubated with alpha,alpha'-dipyridyl was examined, mutant OI molecules melted at a lower temperature than control. These data indicate that the decreased thermal denaturation temperature of OI mutant collagen is independent of post-translational overmodification of lysine or hydroxylysine. Presumably, substitutions for glycine in the Gly-X-Y structural motif distort the helix and produce lower melting temperatures by presently unknown mechanisms.

Glyburide blocks the relaxation response to BRL 34915 (cromakalim), minoxidil sulfate and diazoxide in vascular smooth muscle

BRL 34915 [6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl) 2H-benzo(b) pyran-3-ol], minoxidil sulfate and diazoxide may relax vascular smooth muscle via hyperpolarization due to an opening of membrane potassium channels. We therefore examined the effects of several potassium channel antagonists on the relaxation response to these vasodilators in isolated rat portal venous strips which were mounted in vitro for detecting changes in isometric force. BRL 34915 (IC50 = 4.7 X 10(-8) M), minoxidil sulfate (IC50 = 1.4 X 10(-7) M) and diazoxide (IC50 = 5 X 10(-6) M) elicited concentration-dependent relaxations of the spontaneous, myogenic contractions in venous strips. The relatively nonselective potassium channel antagonists tetraethylammonium ion (0.3-10 X 10(-3) M) and 4-aminopyridine (1-10 X 10(-3) M) caused concentration-dependent shifts (5- to 50-fold) in the relaxation responses to each vasodilator. Charybdotoxin (up to 10(-7) M) and apamin (up to 10(-7) M), known to be antagonists of high and low conductance calcium-activated potassium channels, respectively, had no inhibitory effect on the relaxation-response curves to BRL 34915, minoxidil sulfate or diazoxide. Glyburide (10(-7) to 3 X 10(-5) M), a sulfonylurea which has been shown to block the ATP-modulated potassium channel in insulin-secreting cells, caused concentration-dependent shifts to the right (up to 100-fold) of the IC50 value for BRL 34915 and diazoxide, and at 10(-6) M, abolished the relaxation response to minoxidil sulfate.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of minoxidil on the development of the chick embryo

The effects of minoxidil were studied on chick embryos of 24 and 48 hours of incubation. Minoxidil (3%) was injected into the air sacs of the eggs at doses of 20, 30, 40, and 50 microliters per egg. The controls received 100 microliters of physiological saline. All the embryos, including controls, were examined at day 13. The total number of eggs used in this study was 300. At 24 hours incubation, the percentage of survival ranged from 87 to 21 as the dosages of minoxidil were increased from 20 microliters to 50 microliters per egg (controls = 87%). The survival of the embryos ranged from 79% to 9% after the 48-hour treatment with the similar dosages of minoxidil utilized for the 24-hour group (controls = 83%). A low incidence of gross malformations such as twisted limbs, abnormal beak, short neck and everted viscera were observed; however, the increased incidence was not statistically significant when compared to controls. Body hemorrhage and edema were of high occurrence among the treated embryos. These effects are probably secondary to the known pharmacological effects of minoxidil. The frequency and types of gross malformations did not vary much in the 24 or 48-hour treated groups.

Alveolar hypoxia, inhibition of hypoxic pulmonary vasoconstriction, and permeability edema

We previously reported that regional alveolar hypoxia reduces oleic acid-induced permeability edema formation [Cheney et al. (1987). J. Appl. Physiol. 62: 1690-1697]. In order to define the role of hypoxic pulmonary vasoconstriction (HPV) on this effect, we studied the effects of regional alveolar hypoxia on permeability edema formation with this response inhibited. Dogs weighing 25 +/- 1 kg in which the HPV response had been inhibited by the administration of minoxidil (1 mg/kg i.v.) were anesthetized, mechanically ventilated and had a bronchial divider placed so the left lower lobe (LLL) could be ventilated with an FIO2 = 0.05 or FIO2 = 1, while the right lung was continuously ventilated with an FIO2 = 1.0. In 10 study animals the LLL was ventilated with an FIO2 = 0.05 for 4 h after induction of bilateral permeability pulmonary edema with 0.05 ml/kg of intravenous oleic acid. In six control animals the LLL was ventilated with an FIO2 = 1 for 4 h after the same injury. Postmortem gravimetric analysis indicates that alveolar hypoxia of the LLL with the HPV response inhibited had no effect on pulmonary edema formation. We conclude that inhibition of HPV abolishes the protective effect of regional alveolar hypoxia on oleic acid-induced permeability edema formation.

The measurement of norepinephrine clearance and spillover rate into plasma in conscious spontaneously hypertensive rats

The clearance of norepinephrine from plasma and the spillover rate of norepinephrine into plasma were determined in conscious unrestrained spontaneously hypertensive rats by measuring the concentrations of 3H-norepinephrine and norepinephrine in arterial plasma after 90 min of i.v. infusion with 3H-norepinephrine. In 50 conscious spontaneously hypertensive rats treated with saline (control animals), the following basal values were obtained: plasma norepinephrine concentration = 149 +/- 5 pg/ml; plasma epinephrine concentration = 61 +/- 4 pg/ml; norepinephrine clearance = 188 +/- 4 ml min-1 kg-1; and norepinephrine spillover rate = 27.5 +/- 0.8 ng min-1 kg-1. A significant portion of infused 3H-norepinephrine appeared to be cleared from the plasma by the uptake1 process, since desipramine decreased norepinephrine clearance by 32%. The vasodilating agents hydralazine and minoxidil produced dose-related increases in norepinephrine spillover rate and plasma norepinephrine concentration, but the percent increases in norepinephrine spillover rate exceeded the percent increases in plasma norepinephrine concentration because of concomitant increases in norepinephrine clearance, particularly after treatment with minoxidil. The increase in norepinephrine clearance caused by hydralazine and minoxidil probably resulted from the increase in cardiac output and resultant increase in hepatic and/or pulmonary blood flow. Adrenal secretion of norepinephrine did not appear to contribute to the elevation in norepinephrine spillover rate elicited by hydralazine and minoxidil. Chlorisondamine suppressed norepinephrine spillover rate by 77%, in association with a 70% decline in plasma epinephrine concentration, whereas bretylium lowered norepinephrine spillover rate by only 41%, with no change in plasma epinephrine concentration. The decrements in norepinephrine clearance caused by chlorisondamine (-23%) and bretylium (-15%) were more or less proportional to the magnitude of the vasodepression caused by these drugs. Both norepinephrine spillover rate and clearance fell in a dose-related fashion after treatment with clonidine. After treatment with the sympathoinhibitory agents chlorisondamine, bretylium and clonidine, the percent decreases in norepinephrine spillover rate always exceeded the percent decreases in plasma norepinephrine concentration. Based on these observations, we conclude that norepinephrine spillover rate provides a more accurate measurement of the activity of the peripheral sympathetic nervous system than does plasma norepinephrine concentration in conscious spontaneously hypertensive rats.

Topical minoxidil: cardiac effects in bald man

Systemic cardiovascular effects during chronic treatment with topical minoxidil vs placebo were evaluated using a double-blind, randomized design for two parallel groups (n = 20 for minoxidil, n = 15 for placebo). During 6 months of follow-up, blood pressure did not change, whereas minoxidil increased heart rate by 3-5 beats min-1. Compared with placebo, topical minoxidil caused significant increases in LV end-diastolic volume, in cardiac output (by 0.751 min-1) and in LV mass (by 5 g m-2). We conclude that in healthy subjects short-term use of topical minoxidil is likely not to be detrimental. However, safety needs to be established regarding ischaemic symptoms in patients with coronary artery disease as well as for the possible development of LV hypertrophy in healthy subjects during years of therapy.