Effects of vasoactive topical agents on the survival of dorsal skin flaps in rats

OBJECTIVE

To evaluate 3 vasoactive topical agents regarding their ability to improve the survival of random skin flaps in rats.

METHOD

Thirty-five adult Sprague-Dawley rats had a single 9 x 2 cm random skin flap raised with topical application of a white petroleum ointment (control group), prostaglandin E2, minoxidil, or nitroglycerin to the flap. The effect of the agents on the viability of the flaps was evaluated.

RESULTS

The control group had an average survival area of 60.8% +/- 7.9%. The nitroglycerin and minoxidil groups were not significantly different, with an average area of survival of 60.1% +/- 8.1% and 63.3% +/- 4.7%, respectively. The prostaglandin E2 group had average area of survival of 74.2% +/- 10.5% (P = 0.002).

CONCLUSION

Prostaglandin E2 demonstrated a significant increase in flap survival. Nitroglycerin ointment and minoxidil had no beneficial effects when compared with control.

Minoxidil increases 17 beta-hydroxysteroid dehydrogenase and 5 alpha-reductase activity of cultured human dermal papilla cells from balding scalp

Minoxidil is known to induce hair growth in male pattern baldness, for which development androgen plays a central role. We studied the effect of minoxidil on testosterone metabolism by cultured dermal papilla cells from balding or nonbalding scalp and dermal fibroblasts. In all three groups, 17beta-hydroxysteroid dehydrogenase activity was much higher than 5alpha-reductase activity. Minoxidil increased 17beta-hydroxysteroid dehydrogenase activity by nearly 40% (P < 0.001) in dermal papilla cells of balding scalp, whereas the effect was less marked in dermal papilla cells from nonbalding scalp and dermal fibroblasts. 5alpha-Reductase activity was also slightly increased by minoxidil in dermal papilla cells from balding scalp. Again, the effect on 5alpha-reductase activity was insignificant in the other two groups of cells. Whether such modification of testosterone metabolism in dermal papilla cells of balding scalp by minoxidil is related to its therapeutic effect remains unknown.

The induction by topical minoxidil of increased fenestration in the perifollicular capillary wall

We report the induction by topical minoxidil of increased fenestration in the perifollicular capillary wall. Male 30-day-old Wistar rats were divided into two groups, i.e. an experimental group which received 4% minoxidil solution topically on the dorsal skin, and a control group which received only vehicle solution topically. Using transmission electron microscopy, follicular and subepidermal capillaries and dermal fibres were compared between both groups. There were no obvious differences in subepidermal capillaries or dermal fibres between the two groups. However, topically applied minoxidil increased fenestration in follicular capillary walls around anagen hair bulbs.

Quinine-induced inhibition of gastrointestinal transit in mice: possible involvement of endogenous opioids

The effect of quinine, a cinchona alkaloid, was studied on gastrointestinal transit in mice. Intraperitoneal (i.p.) administration of quinine inhibited the intestinal propulsion of a charcoal suspension at a dose of 100 mg/kg, comparing favorably with 5 mg/kg morphine. In an attempt to probe into the mechanism underlying this inhibition, a possible modulation by minoxidil (1 mg/kg, p.o.) and glibenclamide (1 mg/kg, p.o.), the drugs that, respectively, open and close ATP-sensitive K+ channels was tested on gastrointestinal transit in animals treated or not with quinine or morphine. While minoxidil produced no significant change of normal transit, glibenclamide significantly increased it. However, both drugs blocked the quinine-induced reduction in gastrointestinal transit. In contrast, the inhibitory effect of morphine on gastrointestinal transit was not modified by either drug. The effects of quinine as well as of morphine on gastrointestinal transit were significantly antagonized by naloxone (2 mg/kg, s.c.), a mu-opioid receptor antagonist but not by yohimbine (1 mg/kg, i.p.), an alpha2-adrenoceptor antagonist. Furthermore, quinine at a lower dose (25 mg/kg) that showed no per se effect on gastrointestinal transit, significantly potentiated the response to 2.5 mg/kg morphine. Although the role of ATP-sensitive K+ channels in the action of quinine and morphine was not clarified by the present results, a possible involvement of endogenous opioid(s) in the quinine-induced inhibition of gastrointestinal transit can be suggested.

Pressure mediates angiotensin II-induced arterial hypertrophy and PDGF-A expression

Angiotensin II (Ang II) may induce arterial hypertrophy either directly or through an increase in arterial pressure. To separate these 2 mechanisms, rats were implanted with osmopumps delivering either Ang II (100 ng x kg-1 x min-1) or saline. 5-Bromo-2'-deoxyuridine (BrdU) was delivered to both groups by osmopump (2.5 microg x kg-1 x min-1). Half of the rats in each group were given minoxidil (9 mg x kg-1 . d-1) in their drinking water. After 14 days, systolic blood pressure was 117+/-2, 124+/-3, and 115+/-2 mm Hg in the control, Ang II-minoxidil, and minoxidil groups, respectively, and 181+/-6 mm Hg in the Ang II group (P<0.05). After perfusion-fixation, the thoracic aorta, carotid artery, small mesenteric artery, external spermatic artery, and kidneys were harvested, paraffin-embedded, and used for morphological measurements, immunohistochemistry for BrdU, and in situ hybridization with a 35S-labeled riboprobe for platelet-derived growth factor-A chain (PDGF-A) mRNA. The walls of the aorta and carotid arteries hypertrophied in the Ang II group only. There were no significant morphological differences in the small arteries. BrdU was negative in all arteries but positive in the renal tubules. Expression of PDGF-A was elevated 8-fold in the thoracic aorta of the Ang II group (P<0.05). These results show that (1) arterial hypertrophy from Ang II infusion occurs in response to elevated arterial pressure, (2) hypertrophy was not associated with hyperplasia or polyploidy of vascular smooth muscle cells, and (3) PDGF-A expression correlated with elevated pressure and arterial wall hypertrophy.

Effects of ATP-sensitive potassium channel regulators on chloride channels in the sarcoplasmic reticulum vesicles from rabbit skeletal muscle

The lipid bilayer technique was used to examine the effects of the ATP-sensitive K+ channel inhibitor (glibenclamide) and openers (diazoxide, minoxidil and cromakalim) and Cl- channel activators (GABA and diazepam) on two types of chloride channels in the sarcoplasmic reticulum (SR) from rabbit skeletal muscle. Neither diazepam at 100 microM nor GABA at 150 microM had any significant effect on the conductance and kinetics of the 75 pS small chloride (SCl) channel. Unlike the 150 pS channel, the SCl channel is sensitive to cytoplasmic glibenclamide with Ki approximately 30 microM. Glibenclamide induced reversible decline in the values of current (maximal current amplitude, Imax and average mean current, I') and kinetic parameters (frequency of opening Fo, probability of the channel being open Po and mean open time, To, of the SCl channel. Glibenclamide increased mean closed time, Tc, and was a more potent blocker from the cytoplasmic side (cis) than from the luminal side (trans) of the channel. Diazoxide increased I', Po, and To in the absence of ATP and Mg2+ but it had no effect on Imax and also failed to activate or remove the glibenclamide- and ATP-induced inhibition of the SCl channel. Minoxidil induced a transient increase in I' followed by an inhibition of Imax, whereas cromakalim reduced Po and I' by increasing channel transitions to the closed state and reducing To without affecting Imax. The presence of diazoxide, minoxidil or cromakalim on the cytoplasmic side of the channel did not prevent [ATP]cis or [glibenclamide]cis from blocking the channel. The data suggest that the action(s) of these drugs are not due to their effects on the phosphorylation of the channel protein. The glibenclamide- and cromakalim-induced effects on the SCl channel are mediated via a "flicker" type block mechanism. Modulation of the SCl channel by [diazoxide]cis and [glibenclamide]cis highlights the therapeutic potential of these drugs in regulating the Ca2+-counter current through this channel.

Arterial vasodilation and vascular connective tissue changes in spontaneously hypertensive rats

Arterial hypertrophy in response to hypertension includes increases in the connective tissue proteins elastin and collagen. Regression of arterial hypertrophy depends not only on blood pressure normalization but also on the specific antihypertensive treatment. Consequently, each drug class may exert an influence on connective tissue proteins. We evaluated the arterial connective tissue response of 16-week-old spontaneously hypertensive rats (SHRs) to treatment with minoxidil, 120 mg/L, drinking water for 10 weeks. Despite a decrease in blood pressure, minoxidil had no effect on arterial weight or collagen content but increased elastin content in the abdominal aorta, renal, and superior mesenteric arteries. The increase in elastin content in the abdominal aorta and superior mesenteric artery was accompanied by a decrease in tissue elastase activity. Thus the minoxidil-induced increase in arterial elastin content may be related to a direct effect of the drug to decrease elastase activity in these tissues.

Hair follicle elongation in organ culture of skin from newborn and adult mice

By means of skin organ culture, some biological characteristics of hair follicle elongation were examined. When skin sections from 6-day-old C3H mice were cultured, spontaneous elongation of hair follicles was maintained. Without insulin, hair follicle elongation was poorly maintained irrespective of the presence of serum at 20%. Insulin could be replaced by IGF-I at 100 ng/ml. During in vitro elongation of hair follicles, bromodeoxyuridine was incorporated into germinal epithelial cells around dermal papillae. Skin sections from 4-week-old C3H mice did not show hair follicle elongation in complete medium. However, when 0.5 mM minoxidil was added to the medium, concentration dependent thickening and elongation of hair follicles was observed. In contrast, in vitro elongation of newborn pelage hair follicles was not enhanced by minoxidil. These results suggest that this system with the use of skin sections from 4-week-old C3H mice would be a potential in vitro model of human androgenic alopecia in which the anagen phase is suppressed but the suppression is partially released by minoxidil.

Minoxidil-induced alteration of corneal topography after radial keratotomy

PURPOSE

To determine the antiproliferative effect of minoxidil on human corneal epithelium (hCE) proliferation in vitro and to assess whether topical minoxidil can significantly alter corneal topography after radial keratotomy (RK) by inhibiting myofibroblast activity in the keratotomy wound.

SETTING

Corneal Research Laboratory, University of Chicago, Illinois, USA.

METHODS

In the in vitro evaluation, proliferating hCE was exposed to minoxidil (0.1 to 2.0 mM) for 96 hours to determine the minimum inhibitory dose. Human corneal epithelium cell proliferation was assessed by the incorporation of bromodeoxyuridine (BRDU) into DNA. In the in vivo analysis, eight New Zealand albino rabbits had an eight-incision bidirectional RK on one eye and were divided into two groups. The control eyes (n = 3) received tobramycin and dexamethasone (TobraDex), ciprofloxacin hydrochloride (Ciloxan), and balanced salt solution (BSS) drops four times a day for 3 weeks, while the treatment eyes (n = 5) received TobraDex, Ciloxan, and minoxidil 1.0 mM drops four times daily for 3 weeks. The net change in corneal curvature at 3 weeks was analyzed with corneal topography. Myofibroblast activity in the keratotomy wound was assessed using alpha smooth muscle actin staining techniques.

RESULTS

At concentrations of 1.0 mM and above, minoxidil caused a statistically significant, dose-dependent reduction in hCE cellular proliferation ranging from 29 to 44% (P < .05). Minoxidil (1.0 mM) caused a statistically significant central corneal flattening effect of 4.66 diopters (D) after RK in the treatment eyes compared with 1.11 D in the control eyes (P = .05). Histologically, minoxidil-treated keratotomy wounds lacked cells with contractile elements consistent with myofibroblast differentiation. Corneal epithelial wound healing was similar in both groups.

CONCLUSION

At the appropriate dose, topical minoxidil may be a useful adjunctive treatment that can reduce the number of undercorrections after mini-RK without apparent toxicity to the corneal epithelium.

Echocardiography, a non-invasive method for the investigation of heart morphology and function in laboratory dogs: 2. Effects of minoxidil and quinidine on the left ventricle function

We used echocardiography to investigate the changes in the cardiac function of dogs treated with minoxidil (a vasodilator, administered at doses which can produce mild lesions in the myocardium of the left ventricle) and quinidine (an antiarrhythmic at doses up to 8 times the upper limit of the therapeutic range in dogs). Groups of three beagles received a single administration of minoxidil at doses of 0.5 or 2 mg/kg. Two groups of two dogs received a single administration of quinidine at doses of 80 or 160 mg/kg. Two groups of three control dogs were treated concurrently with the vehicle alone. M-mode echocardiography was performed under two-dimensional echocardiography guidance on three occasions the day before treatment, immediately before dosing and 1, 3 and 24 h after dosing. We measured or calculated end diastolic, end systolic, and stroke volumes (EDV, ESV and SV), fractional shortening (FS), ejection fraction (EF), the percentage of thickening of the septum and of the left ventricle posterior wall (PST and PWT), and the mean and maximal velocities of the left ventricle posterior wall (PWVm and PWVM). At the same time as echocardiography recording, heart rate was measured by cardiac auscultation. Minoxidil produced a marked tachycardia. Less marked increases in heart rate occurred after quinidine. Both compounds were associated with a decrease in ESV and with marked increases in FS, EF, PWVm and PWVM which, in comparison with data for controls, are indicative of an increase in the amplitude and velocity of cardiac contraction. Both drugs also produced a decrease in EDV and consequently there was no increase in SV despite the increased amplitude of ventricular contraction. Cardiac output increased in proportion to the increase in heart rate. Overall, the effects were dose-related and are consistent with the pharmacological properties of the compounds. However, to date these effects have been demonstrated only by invasive methods. To conclude, we have shown that echocardiography allows a non-invasive investigation of the cardiac effects of suprapharmacological doses of antiarrhythmics and of the changes in heart function induced by vasodilators known to cause left ventricular lesions in dogs.

Influence of potassium channel modulators on cognitive processes in mice

1. The effect of i.c.v. administration of different potassium channel openers (minoxidil, pinacidil, cromakalim) and potassium channel blockers (tetraethylammonium, apamin, charybdotoxin, gliquidone, glibenclamide) on memory processes was evaluated in the mouse passive avoidance test. 2. The administration of minoxidil (10 microg per mouse i.c.v.), pinacidil (5-25 microg per mouse i.c.v.) and cromakalim (10-25 microg per mouse i.c.v.) immediately after the training session produced an amnesic effect. 3. Tetraethylammonium (TEA; 1-5 microg per mouse i.c.v.), apamin (10 ng per mouse i.c.v.), charybdotoxin (1 microg per mouse i.c.v.), gliquidone (3 microg per mouse i.c.v.) and glibenclamide (1 microg per mouse i.c.v.), administered 20 min before the training session, prevented the potassium channel opener-induced amnesia. 4. At the highest effective doses, none of the drugs impaired motor coordination, as revealed by the rota rod test, or modified spontaneous motility and inspection activity, as revealed by the hole board test. 5. These results suggest that the modulation of potassium channels plays an important role in the regulation of memory processes. On this basis, the potassium channel blockers could be useful in the treatment of cognitive deficits.

Minoxidil upregulates the expression of vascular endothelial growth factor in human hair dermal papilla cells

The hair follicle dermal papilla which controls hair growth, is characterized in the anagen phase by a highly developed vascular network. We have demonstrated in a previous study that the expression of an angiogenic growth factor called vascular endothelial growth factor (VEGF) mRNA varied during the hair cycle. VEGF mRNA is strongly expressed in dermal papilla cells (DPC) in the anagen phase, but during the catagen and telogen phases. VEGF mRNA is less strongly expressed. This involvement of VEGF during the hair cycle allowed us to determine whether VEGF mRNA expression by DPC was regulated by minoxidil. In addition, the effect of minoxidil on VEGF protein synthesis in both cell extracts and DPC-conditioned medium, was investigated immunoenzymatically. Both VEGF mRNA and protein were significantly elevated in treated DPC compared with controls. DPC incubated with increasing minoxidil concentrations (0.2, 2, 6, 12 and 24 mumol/L) induced a dose-dependent expression of VEGF mRNA. Quantification of transcripts showed that DPC stimulated with 24 mumol/L minoxidil express six times more VEGF mRNA than controls. Similarly, VEGF protein production increases in cell extracts and conditioned media following minoxidil stimulation. These studies strongly support the likely involvement of minoxidil in the development of dermal papilla vascularization via a stimulation of VEGF expression, and support the hypothesis that minoxidil has a physiological role in maintaining a good vascularization of hair follicles in androgenetic alopecia.

Interaction between isoniazid and diverse vasodilators: role of decreased cerebral GABA

OBJECTIVE

To determine if the interaction between isoniazid and hydralazine, consisting of increased hypotension accompanied by bradycardia, occurs with other vasodilators.

METHODS

Blood pressure and heart rate responses to a number of vasodilators were determined in rats under chloralose-urethane, pretreated or not with 250 mg/kg of isoniazid. The influence of this dose of isoniazid on GABA levels in the hypothalamus and pons-medulla was assessed in other groups of rats.

RESULTS

Increased hypotension and bradycardia following i.p. isoniazid were observed with dipyridamole, prazosin, pinacidil and hydralazine given i.v. Bradycardia without increased hypotension appeared with papaverine and verapamil, while increased hypotension with unchanged heart rate was observed with minoxidil and captopril. Isoniazid decreased GABA in the hypothalamus and pons-medulla.

CONCLUSIONS

At the high dose used, isoniazid interacts with various vasodilators, irrespective of their mechanism of action. The interaction could be due to the influence of the drug on GABA levels at cardiovascular regulatory sites.

Cerebral arteriolar dilations by KATP channel activators need L-lysine or L-arginine

We investigated the effects of various amino acids on responses to ATP-sensitive potassium (KATP) channel openers in anesthetized cats equipped with cranial windows. The application of pinacidil by superfusion caused transient vasodilation, whereas there was sustained vasodilation from the application of stationary solution of pinacidil. In the presence of L-arginine or L-lysine, pinacidil by superfusion led to sustained vasodilation, suggesting that the rapid flow of fluid displaced these amino acids from binding on the channel and that such binding was essential for opening the channel. NG-nitro-L-arginine blocked responses to pinacidil, and this blockade was reversed by L-lysine or L-arginine but not by D-arginine, D-lysine, methyl-L-arginine, glycine, L-histidine, dimethylarginine, dimethyl-L-arginine, or hydroxylysine. The blockade of responses to pinacidil induced by glyburide was also reversed completely by L-arginine or L-lysine but not by D-arginine, suggesting that these amino acids act on the sulfonylurea receptor. Hydroxylysine but not methyl-L-lysine, dimethylarginine, or dimethyl-L-arginine blocked responses to pinacidil. The findings show that KATP channels in cerebral arterioles need L-lysine or L-arginine to open in response to agonists.

AT2 receptor stimulation increases aortic cyclic GMP in SHRSP by a kinin-dependent mechanism

In the present study we tested the hypothesis whether an angiotensin AT2 receptor-mediated stimulation of the bradykinin (BK)/nitric oxide (NO) system can account for the effects of AT1 receptor antagonism on aortic cGMP described previously in SHRSP. Adult SHRSP were treated for 4 hours with angiotensin II (ANG II) (30 ng/kg per min IV) or vehicle (0.9% NaCl I.V.). Animals were pretreated with vehicle, losartan (100 mg/kg P.O.), PD 123319 (30 mg/kg I.V.), losartan plus PD 123319, icatibant (500 microg/kg I.V.), N(G)-nitro-L-arginine methyl ester (L-NAME; 1 mg/kg I.V.), or minoxidil (3 mg/kg I.V.). Mean arterial blood pressure (MAP) was continuously monitored over the 4-hour experimental period, and plasma ANG II and aortic cGMP were measured by RIA at the end of the study. ANG II infusion over 4 hours raised MAP by about 20 mm Hg. Losartan alone or losartan plus ANG II as well as minoxidil plus ANG II markedly reduced blood pressure when compared to vehicle-treated or ANG II-treated animals, respectively. Plasma levels of ANG II were increased 2-fold by ANG II infusion alone or by ANG II in combination with icatibant, L-NAME, or minoxidil. The increase in plasma ANG II levels was even more pronounced after losartan treatment. Aortic cGMP content was significantly increased by ANG II, losartan, losartan plus ANG II, and minoxidil plus ANG II by 60%, 45%, 68%, and 52%, respectively (P<.05). The effects of ANG II and of losartan plus ANG II on aortic cGMP content were both blocked by cotreatment with the AT2 receptor antagonist PD 123319. Icatibant and L-NAME abolished the effects of ANG II on aortic cGMP. Our results demonstrate the following: (1) ANG II increases aortic cGMP by an AT2 receptor-mediated action because the effect could be prevented by an AT2 receptor antagonist; (2) the effect of ANG II was not secondary to blood pressure increase because it remained under reduction of MAP with minoxidil; (3) losartan increased aortic cGMP most likely by increasing plasma ANG II levels with a subsequent stimulation of AT2 receptors; and (4) the effects of AT2 receptor stimulation are mediated by BK and, subsequently, NO because they were abolished by B2 receptor blockade as well as by NO synthase inhibition.

Biphasic effects of minoxidil on the proliferation and differentiation of normal human keratinocytes

Minoxidil is the most used drug with proved effects in the treatment of androgenetic alopecia (AGA), but little is known about its pharmacological activity and target cells in hair follicles. As AGA is characterized by follicle atrophy, accelerated hair cycles and hair fiber thinning, we postulated that keratinocyte proliferation/differentiation is affected and we tested Minoxidil's effects on those parameters. Normal human keratinocytes (NHK) of follicular or epidermal origin were cultured in the presence of Minoxidil (0, 0.1, 1, 10, 100, 1,000 microM) during 5-8 days in various media (high-/low-calcium content, with or without serum). Proliferation was assessed by mitochondrial dehydrogenase activity (XTT), BrdU incorporation, lysosome numeration (neutral red incorporation) and total protein dosage. Drug-induced cytotoxicity was measured by lactate dehydrogenase release in culture supernatant, and pro-differentiating effects were evaluated by relative involucrin expression (ELISA dosage). On this basis, we showed that Minoxidil had biphasic effects on the proliferation and differentiation of NHK: Minoxidil stimulated NHK proliferation at micromolar doses, while antiproliferative, pro-differentiative and partially cytotoxic effects were observed with millimolar concentrations. We can hypothesize that Minoxidil hypertrichotic activity in vivo is possibly mediated by the maintenance of proliferative potential in follicular keratinocytes precociously committed to differentiation.

[The anti-anaphylactic action of potassium channel openers and its mechanism]

The anti-anaphylactic action of potassium channel openers was studied and reported in this paper. Minoxidil(Min) was shown to inhibit passive cutaneous anaphylaxis in rats. Diazoxide (Dia) and Min were found to inhibit antigen-induced guinea-pig ileum smooth muscle contraction in vitro. Min was shown to antagonize 5-HT-induced capillary permeability in rat skin. Dia was demonstrated to inhibit histamine release from rat peritoneal mast cells induced by A23187 and compound 48/80, but it failed to antagonize guinea-pig ileum smooth muscle contraction induced by histamine in vitro. These results provide evidence that potassium channel openers may be a new group of inhibitors of histamine release and indicate that the mechanism of its anti-anaphylactic action may be related to its potassium channel opening effect. As a result of this effect, Ca2+ influx to the mast cells decreases and Ca2+ release from calcium storage was inhibited, thus inhibiting histamine release.

Binding and effects of KATP channel openers in the vascular smooth muscle cell line, A10

1. The ATP-sensitive K+ channel (KATP channel) in A10 cells, a cell line derived from rat thoracic aorta, was characterized by binding studies with the tritiated KATP channel opener, [3H]-P1075, and by electrophysiological techniques. 2. Saturation binding experiments gave a KD value of 9.2 +/- 5.2 nM and a binding capacity (BMax) of 140 +/- 40 fmol mg-1 protein for [3H]-P1075 binding to A10 cells; from the BMax value a density of binding sites of 5-10 per microns2 plasmalemma was estimated. 3. KATP channel modulators such as the openers P1075, pinacidil, levcromakalim and minoxidil sulphate and the blocker glibenclamide inhibited [3H]-P1075 binding. The extent of inhibition at saturation depended on the compound, levcromakalim inhibiting specific [3H]-P1075 binding by 85%, minoxidil sulphate and glibenclamide by 70%. The inhibition constants were similar to those determined in strips of rat aorta. 4. Resting membrane potential, recorded with microelectrodes, was -51 +/- 1 mV. P1075 and levcromakalim produced a concentration-dependent hyperpolarization by up to -25 mV with EC50 values of 170 +/- 40 nM and 870 +/- 190 nM, respectively. The hyperpolarization induced by levcromakalim (3 microM) was completely reversed by glibenclamide with an IC50 value of 86 +/- 17 nM. 5. Voltage clamp experiments were performed in the whole cell configuration under a physiological K+ gradient. Levcromakalim (10 microM) induced a current which reversed around -80 mV; the current-voltage relationship showed considerable outward rectification. Glibenclamide (3 microM) abolished the effect of levcromakalim. 6. Analysis of the noise of the levcromakalim (10 microM)-induced current at -40 and -20 mV yielded estimates of the channel density, the single channel conductance and the probability of the channel to be open of 0.14 micron-2, 8.8 pS and 0.39, respectively. 7. The experiments showed that A10 cells are endowed with functional KATP channels which resemble those in vascular tissue; hence, these cells provide an easily accessible source of channels for biochemical and pharmacological studies. The density of binding sites for [3H]-P1075 was estimated to be one order of magnitude higher than the density of functional KATP channels; assuming a plasmalemmal localization of the binding sites this suggests a large receptor reserve for the openers in A10 cells.

Evidence for alpha 2-adrenoceptor agonist activity of minoxidil

The present investigation was undertaken to study the mechanism of action of minoxidil using various smooth muscle preparations. Minoxidil (4.7 x 10(-6) M to 4.7 x 10(-4) M) produced a concentration-dependent inhibition of field stimulation-evoked responses in rat anococcygeus muscle and vas deferens. The inhibition produced by minoxidil was antagonized by yohimbine (2.5 x 10(-7) M). Minoxidil (1.4 x 10(-5) M to 4.7 x 10(-4) M) also produced a concentration-dependent relaxation in oestrogen-primed potassium chloride-depolarized rat uterus. These responses were blocked not only by yohimbine but also by glibenclamide (2.02 x 10(-8) M). Our results suggest that minoxidil possesses alpha 2-adrenoceptor agonist activity in addition to potassium-channel-opening activity.

Minoxidil and wound contraction

Minoxidil has been proposed as a potential topical inhibitor of wound contraction and proliferative scarring. Suggestions for this application are derived from in vitro investigations demonstrating inhibition of various fibroblastic functions. The purpose of this study was to attempt to establish in vivo support of these effects using an established animal model of wound contraction. Standardized cutaneous wounds were created on the dorsum of Sprague-Dawley rats, which were divided equally into six treatment groups. Wounds were treated daily after tracing their unhealed areas. On complete closure of the wounds, analyses of the contraction rates and tensile strength were performed for comparison among groups. Minoxidil did not demonstrate significant inhibition of wound contraction rates relative to either an inert vehicle, an active vehicle, or no treatment. Contrarily, as previously demonstrated in this animal model, silver sulfadiazine did demonstrate significant inhibition of wound contraction rates relative to both vehicles. No significant difference in tensile strength was demonstrated among groups. These observations do not support the proposed use of minoxidil as an "antifibrotic" agent.

Effect of K+ channel openers on K+ channel in cultured human dermal papilla cells

Minoxidil sulfate and pinacidil, well-known activators of the ATP-sensitive K+ (KATP) channel, induce hair growth in clinical studies. The opening of K+ channels is thought to be an important mechanism in the regulation of hair follicles. In the present study, we used the patch clamp technique to characterize the K+ channels and tested the effect of K+ channel openers on K+ channels in cultured human dermal papilla cells. In dermal papilla cells, the Ca(2+)-activated K+ (KCa) channel with large conductance (179.3 +/- 13.1 pS in symmetrical 150 mM K+ solutions, n = 9) was dominant and we could not observe KATP channels in cell-attached and inside-out patches. In addition, minoxidil and pinacidil failed to activate KATP or KCa channels. In inside-out membrane patches, the channel was blocked by 10 mM tetraethylammonium ion, 2 mM 4-aminopyridine to the cytosolic face of the membrane or by lowering Ca2+ using 10 mM EGTA, but not by glibenclamide. In the cell-attached patch configurations, extracellular application of 1 mM sodium nitroprusside, a nitrovasodilator, activated the KCa channel. Methylene blue (2 mM) inhibited channel activation by sodium nitroprusside. Extracellular application of 20 mM dibutyryl cGMP activated the KCa channel, suggesting that channel activation is mediated by cGMP. Nitrovasodilators, which have no effect on hair growth, now appear to activate KCa channels in dermal papilla cells. These results suggest that increased K+ permeability itself in dermal papilla cells may not be sufficient for promotion of hair growth.

Hypoxia-induced catecholamine release and intracellular Ca2+ increase via suppression of K+ channels in cultured rat adrenal chromaffin cells

Hypoxia (5% O2) enhanced catecholamine release in cultured rat adrenal chromaffin cells. Also, the intracellular free Ca2+ concentration ([Ca2+]i) increased within 3 min in approximately 50% of the chromaffin cells under hypoxic stimulation. The increase depended on the presence of extracellular Ca2+. Nifedipine and omega-conotoxin decreased the population of the cells that showed the hypoxia-induced [Ca2+]i increase, showing that the Ca2+ influx was attributable to L- and N-type voltage-dependent Ca2+ channels. The membrane potential was depolarized during the perfusion with the hypoxic solution and returned to the basal level following the change to the normoxic solution (20% O2). Membrane resistance increased twofold under the hypoxic condition. The current-voltage relationship showed a hypoxia-induced decrease in the outward K+ current. Among the K+ channel openers tested, cromakalim and levcromakalim, both of which interact with ATP-sensitive K+ channels, inhibited the hypoxia-induced [Ca2+]i increase and catecholamine release. The inhibitory effects of cromakalim and levcromakalim were reversed by glibenclamide and tolbutamide, potent blockers of ATP-sensitive K+ channels. These results suggest that some fractions of adrenal chromaffin cells are reactive to hypoxia and that K+ channels sensitive to cromakalim and glibenclamide might have a crucial role in hypoxia-induced responses. Adrenal chromaffin cells could thus be a useful model for the study of oxygen-sensing mechanisms.

Effect of K+ channel modulation on mouse feeding behaviour

The K+ channel antagonists, glucose (100 microg per mouse i.c.v.), tetraethylammonium (1 microg per mouse i.c.v.) and apamin (1 ng per mouse i.c.v.), reduced food intake of mice comparably to the two anorectic drugs, amphetamine (10 microg per mouse i.c.v.) and cocaine (50 microg per mouse i.c.v.). Conversely, the K+ channel openers, minoxidil (5 microg per mouse i.c.v.) and pinacidil (10 microg per mouse i.c.v.), elicited an orectic effect of the same intensity as that induced by 2-deoxyglucose (200 microg per mouse i.c.v.), aurothioglucose (200 microg per mouse i.c.v.) and neuropeptide Y (0.5 microg per mouse i.c.v.). The antisense oligodeoxyribonucleotide (1-3 nmol per injection) to mKv1.1 gene produced, at 72 h, a dose-dependent increase in food intake. A quantitative reverse transcription-polymerase chain reaction (RT-PCR) study demonstrated a reduction in cerebral mRNA levels only in the antisense oligodeoxyribonucleotide-treated group, indicating the absence of a sequence-independent action. Mice receiving the K+ channel modulators or antisense oligodeoxyribonucleotide had unmodified motor coordination and inspection activity as revealed, respectively, by the rotarod and hole-board tests. The integrity and functionality of central K+ channels appears, therefore, to be fundamental in the regulation of food intake by mice.

Coadministration of glyburide and minoxidil, drugs with opposing effects on potassium channels

INTRODUCTION

Adenosine triphosphate (ATP)-sensitive potassium (K+) channels are modulated by drugs, so that they are opened by vasodilators such as minoxidil but are closed by hypoglycemic agents such as glyburide (glibenclamide). Animal studies and in vitro evidence suggests that the coadministration of drugs with opposing effects on K+ channels attenuates their pharmacodynamic effects.

METHODS

To investigate whether this important pharmacodynamic interaction occurs in humans, we administered 5 mg minoxidil, 2.5 mg glyburide or both in a double-blind fashion to nine healthy subjects. Glucose and insulin responses during an intravenous glucose tolerance test (0.3 gm/kg) were measured and blood pressure was recorded for 8 hours. In an additional four subjects the effect of 5 mg glyburide on the hypotensive effect of 5 mg minoxidil was examined.

RESULTS

None of the parameters of glucose metabolism differed significantly when subjects received glyburide alone, minoxidil alone, or glyburide with minoxidil. Minoxidil or minoxidil in combination with 2.5 mg glyburide resulted in a similar significant decrease in blood pressure compared with the response to glyburide alone. The hypotensive effect of minoxidil was smaller in the four subjects who received the higher dose of glyburide, but significant hypoglycemia (blood glucose concentration < 60 mg/dl) occurred in three of the four subjects.

CONCLUSION

We conclude that, in healthy volunteers, the coadministration of 2.5 mg glyburide and 5 mg minoxidil does not result in attenuation of the blood pressure-lowering effect of minoxidil. The smaller hypotensive response in four subjects who received 5 mg glyburide and 5 mg minoxidil suggests the possibility of a dose-related drug interaction. Studies with strict clamping of blood glucose concentrations will be required to address this possibility.

Activation of cytoprotective prostaglandin synthase-1 by minoxidil as a possible explanation for its hair growth-stimulating effect

Data from the literature indicate that nonsteroidal anti-inflammatory drugs (NSAIDs), such as indomethacin, naproxen, piroxicam, or ibuprofen, induce hair loss in vivo. These NSAIDs are well-known inhibitors of both the cytoprotective isoform of prostaglandin endoperoxide synthase-1 (PGHS-1) and of the inducible form (PGHS-2). By immunohistochemical staining, we found that PGHS-1 is the main isoform present in the dermal papilla from normal human hair follicle (either anagen or catagen), whereas PGHS-2 was only faintly and exclusively expressed in anagen dermal papilla. Thus, PGHS-1 might be the primary target of the hair growth-inhibitory effects of NSAIDs. We thus speculated that activation of PGHS-1 might be a mechanism by which minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxyde) stimulates hair growth in vivo. We demonstrate here that minoxidil is a potent activator of purified PGHS-1 (AC50 = 80 microM), as assayed by oxygen consumption and PGE2 production. This activation was also evidenced by increased PGE2 production by BALB/c 3T3 fibroblasts and by human dermal papilla fibroblasts in culture. Our findings suggest that minoxidil and its derivatives may have a cytoprotective activity in vivo and that more potent second-generation hair growth-promoting drugs might be designed, based on this mechanism.