The effect of polyamines on KATP channels in guinea-pig ventricular myocytes

Functional K(ATP) channels in the rat retinal microvasculature: topographical distribution, redox regulation, spermine modulation and diabetic alteration

The essential task of the circulatory system is to match blood flow to local metabolic demand. However, much remains to be learned about this process. To better understand how local perfusion is regulated, we focused on the functional organization of the retinal microvasculature, which is particularly well adapted for the local control of perfusion. Here, we assessed the distribution and regulation of functional K(ATP) channels whose activation mediates the hyperpolarization induced by adenosine. Using microvascular complexes freshly isolated from the rat retina, we found a topographical heterogeneity in the distribution of functional K(ATP) channels; capillaries generate most of the K(ATP) current. The initiation of K(ATP)-induced responses in the capillaries supports the concept that the regulation of retinal perfusion is highly decentralized. Additional study revealed that microvascular K(ATP) channels are redox sensitive, with oxidants increasing their activity. Furthermore, the oxidant-mediated activation of these channels is driven by the polyamine spermine, whose catabolism produces oxidants. In addition, our observation that spermine-dependent oxidation occurs predominately in the capillaries accounts for why they generate most of the K(ATP) current detected in retinal microvascular complexes. Here, we also analysed retinal microvessels of streptozotocin-injected rats. We found that soon after the onset of diabetes, an increase in spermine-dependent oxidation at proximal microvascular sites boosts their K(ATP) current and thereby virtually eliminates the topographical heterogeneity of functional K(ATP) channels. We conclude that spermine-dependent oxidation is a previously unrecognized mechanism by which this polyamine modulates ion channels; in addition to a physiological role, spermine-dependent oxidation may also contribute to microvascular dysfunction in the diabetic retina.

Spermine-induced negative inotropic effect in isolated rat heart, is mediated through the release of ATP

Putrescine, spermidine and spermine are natural compounds found in up to millimolar concentrations in eukaryotic and prokaryotic cells. At physiologic pH, the polyamines are protonated (+2, +3 and +4 charges), their polycationic properties lead to the assumption that they could affect physiological systems by binding to anionic sites of the cellular membrane and/or by modulating ion channels. At the cardiovascular level, their effects are not completely understood. However, these compounds may be able to exert the induction of synthesis and release of cellular mediators. In an attempt to explore this possibility, we used the isolated and perfused rat heart, Langendorff, model in order to evaluate the inotropic effects of these polyamines, putrescine, spermidine and spermine. Dose-response curves (0.1-0.6 mM) for putrescine, spermidine and spermine were constructed; with the finding that spermine had the largest negative effect. The obtained effects were not blocked by nitric oxide synthesis inhibitors (L-NAME), H(1) and H(2) receptor antagonists (Brompheniramine and Cimetidine) or by Glibenclamide, an antagonist of ATP-sensitive K(+) channels. We found that spermine-induced and increased ATP concentration in cardiac effluents. Reactive Blue, a P(2y) purinoreceptor antagonist and Aminophylline, an unspecific adenosine receptor antagonist, blocked the spermine-induced effects. These results showed that ATP, at least in part, is responsible of the spermine cardiovascular effects. Adenosine was shown to also play an important role on those effects.

Polyamines as olfactory stimuli in the goldfish Carassius auratus

Electrophysiological responses of goldfish olfactory receptor neurons (ORNs) and goldfish behavioral responses to polyamines were investigated in vivo. Electro-olfactogram (EOG) recordings indicated that polyamines (putrescine, cadaverine and spermine) are potent olfactory stimuli for goldfish with estimated electrophysiological thresholds of 10(-8)-10(-7) mol l(-1), similar to that for L-arginine, the most stimulatory amino acid. Although thresholds were similar, the magnitude of the EOG responses to intermediate (10(-5)-10(-4) mol l(-1)) and high (10(-3) mol l(-1)) concentrations of polyamines dwarfed the responses to amino acids and related single amine containing compounds (amylamine and butylamine). The EOG responses to 0.1 mmol l(-1) putrescine, cadaverine and spermine were, respectively, 4.2x, 4.3x and 10.3x the response of the standard, 0.1 mmol l(-1) L-arginine. Electrophysiological cross-adaptation experiments indicated that polyamine receptor sites are independent from those to L-amino acids (alanine, arginine, glutamate, lysine, methionine and ornithine), bile salts (sodium taurocholate and taurolithocholate), the single amine containing compounds (amylamine and butylamine) and ATP. Further, the cross-adaptation experiments revealed the existence of independent receptor sites for the different polyamines tested. Pharmacological experiments suggested that polyamine odorant transduction does not primarily involve the cyclic AMP and IP(3) second messenger pathways. Behavioral assays indicated that polyamines are attractants that elicit feeding behavior similar to that elicited by L-amino acids.

Effects of dietary L-arginine on structure and function of flow-restricted vein grafts

PURPOSE

Experiments were designed to determine effects of dietary supplementation with L -arginine on structure and function of flow-restricted vein grafts.

METHODS

Saphenous veins were placed as bilateral interposition grafts in femoral arteries of two groups of adult male mongrel dogs; one group was maintained on a normal diet (control), the other group supplemented with L -arginine (200 mg/kg per day) beginning 1 week before surgery. In each dog, flow was reduced by 50% in one graft by placing an adjustable clamp on the artery distal to the distal anastomosis. Plasma amino acids and oxidized products of nitric oxide (NO(x )) were measured before and after L -arginine feeding. At postoperative week 4, grafts were removed and prepared for organ chamber studies to determine functions of the endothelium or smooth muscle and for histology.

RESULTS

Plasma L -arginine increased within 3 hours after feeding and increased from 141 +/- 8 nmol/mL to 169 +/- 11 nmol/mL (n = 6) after 5 weeks of supplementation. Plasma ornithine and citrulline paralleled arginine, whereas circulating NO(x ) was unchanged. Maximal contractions to 60 mmol/L KCl were reduced in grafts from L -arginine-fed dogs. Endothelium-dependent relaxations to the calcium ionophore A23187 and relaxations of the smooth muscle NO were reduced in grafts from L -arginine-fed dogs. Neointimal hyperplasia was increased in grafts with reduced flow and not affected by arginine feeding.

CONCLUSIONS

Dietary supplementation with L -arginine did not increase plasma NO in dogs with peripheral vein grafts or increase endothelium-dependent relaxations in control or flow-restricted grafts. Therefore, dietary supplementation with L -arginine may not improve long-term functions of flow-restricted peripheral bypass grafts.

Modulation of potassium channels in the hearts of transgenic and mutant mice with altered polyamine biosynthesis

Inward rectification of cardiac I(K1)channels was modulated by genetic manipulation of the naturally occurring polyamines. Ornithine decarboxylase (ODC) was overexpressed in mouse heart under control of the cardiac alpha -myosin heavy chain promoter (alpha MHC). In ODC transgenic hearts, putrescine and cadaverine levels were highly elevated ( identical with 35-fold for putrescine), spermidine was increased 3.6-fold, but spermine was essentially unchanged. I(K1)density was reduced by identical with 38%, although the voltage-dependence of rectification was essentially unchanged. Interestingly, the fast component of transient outward (I(to,f)) current was increased, but the total outward current amplitude was unchanged. I(K1)and I(to)currents were also studied in myocytes from mutant Gyro (Gy) mice in which the spermine synthase gene is disrupted, leading to a complete loss of spermine. I(K1)current densities were not altered in Gy myocytes, but the steepness of rectification was reduced indicating a role for spermine in controlling rectification. Intracellular dialysis of myocytes with putrescine, spermidine and spermine caused reduction, no change and increase of the steepness of rectification, respectively. Taken together with kinetic analysis of I(K1)activation these results are consistent with spermine being a major rectifying factor at potentials positive to E(K), spermidine dominating at potentials around and negative to E(K), and putrescine playing no significant role in rectification in the mouse heart.

Potassium inhibition of sodium-activated potassium (K(Na)) channels in guinea-pig ventricular myocytes

N(a+)-activated potassium channels (K(Na) channels) were studied in inside-out patches from guinea-pig ventricular myocytes at potentials between -100 and +80 mV. External K(+) (K(+)(o)) was set to 140 mM. For inwardly directed currents with 105 mM internal K(+) (K(+)(1)), the unitary current-voltage relationship was fitted by the constant field equation with a potassium permeability coefficient, P(K), of 3.72 x 10(-13) cm(3) s(-1). The slope conductance (-100 to -10 mV) was 194 +/- 4.5 pS (mean +/- s.d., n = 4) with 105 mM K(+)(i) (35 mM Na(+)(i)) but it decreased to 181 +/- 5.6 pS (n = 5) in 70 mM K(+)(i) (70 mM Na(+)(i)). K(Na) channels were activated by internal Na(+) in a concentration-dependent fashion. With 4 mM K(+)(i), maximal activation was recorded with 100 mM Na(+)(i) (open probability, P(o), about 0.78); half-maximal activation required about 35 mM Na(+)(i). When K(+)(i) was increased to 70 mM, half-maximal activation shifted to about 70 mM Na(+)(i). With Na(+)(i) set to 105 mM, channel activity was markedly inhibited when K(+)(i) was increased from 35 to 105 mM. Channel openings were abolished with 210 mM K(+)(i). The inhibitory effect of internal K(+) was also observed at more physiological conditions of osmolarity, ionic strength and chloride concentration. With 35 mM Na(+)(i) and 4 mM K(+)(i), P(o) was 0.48 +/- 0.10 (n = 6); when K(+)(i)was increased to 35 mM, P(o) was reduced to 0.04 +/- 0.05 (n = 7, P < 0.001). The relationship between P(o) and Na(+)(i) concentration at different levels of K(+)(i) is well described by a modified Michaelis-Menten equation for competitive inhibition; the Hill coefficients were 4 for the P(o)-Na(+)(i) relationship and 1.2 for the P(o)-K(+)(i) relationship. It is suggested that Na(+) and K(+) compete for a superficial site on the channel's permeation pathway. K(Na) channels would be most likely to be activated in vivo when an increase in Na(+)(i) is accompanied by a decrease of K(+)(i).

Propafenone blocks ATP-sensitive K+ channels in rabbit atrial and ventricular cardiomyocytes

Propafenone, a class I antiarrhythmic agent, inhibits several membrane currents (I(Na), I(Ca), I(K), Ito), however, its effects on ATP-sensitive potassium current (I(K)ATP) of cardiac cells have not been tested. We evaluated the blocking effects of 0.1 to 100 microM propafenone applications at 35 degrees C on the whole-cell I(K)ATP as triggered by dinitrophenol (75 microM) in adult rabbit dissociated atrial and ventricular cardiomyocytes in comparison. The block of I(K)ATP by propafenone was dose-dependent, fully reversible and voltage-independent. The dose-response relation, as evaluated at 0 mV for atrial myocytes (ED50 = 1.26+/-0.17 microM, Hill number = 1.25+/-0.22) was significantly shifted to the left vs. that in ventricular myocytes (ED50 = 4.94+/-0.59 microM, Hill number = 1.22+/-0.14). It is concluded that propafenone blocks cardiac I(K)ATP at a single site with 4 times higher affinity for the drug in atrial myocytes. This block of cardiac I(K)ATP might play a role in the beneficial and adverse effects of the drug.