Influence of ATP-sensitive K+ channel modulation on the mechanical properties of diabetic myocardium.

ATP-sensitive K+ (KATP) channels are therapeutic targets for hypertension and diabetes. KATP channel opening elicits vasorelaxation and myocardial protection, whereas its closing stimulates insulin secretion. The cardiac KATP conductance is believed altered under diabetes. This study was to evaluate the influence of KATP channel openers and blocker on myocardial contractile dysfunction in diabetes. Adult rats were made diabetic with streptozotocin (55 mg/kg) and maintained for eight weeks. Contractile properties were studied using isolated papillary muscles in the absence or presence of KATP channel openers (BRL 38227 and pinacidil) and KATP blocker (glyburide). Experimental diabetes led to hyperglycemia, reduced growth, cardiac hypertrophy and hepatomegaly. Mechanical properties exhibited prolonged duration and reduced velocity of both contraction and relaxation in diabetic myocardium, characteristic of diabetic cardiomyopathy. Acute exposure to both KATP channel openers induced concentration-dependent negative inotropic effects (NIE) on myocardial contraction. The magnitude of the NIE was similar between the normal and diabetic groups and was fully reversible upon washout for BRL 38227 although not for pinacidil. Both KATP channel openers depressed the velocity of contraction and relaxation, whereas exerted no effect on the duration of contraction and relaxation, in myocardium from both groups. Acute exposure to glyburide, a KATP channel blocker, failed to alter any of the mechanical parameters measured. These data suggest that acute modulation of KATP channel with channel opener or blocker had little influence on diabetic cardiomyopathy, at least in the setting of multicellular preparations.

Influence of age on the inotropic response to acute ethanol exposure in spontaneously hypertensive rats.

Acute ethanol exposure depresses cardiac electromechanical function, whereas chronic ethanol consumption leads to the development of a specific myopathic state. Chronic hypertension and aging have similar effects in the impairment of myocardial function. However, little is known about the effects of ethanol on cardiac mechanical function in hypertension. We studied the effect of age on baseline mechanical properties and the inotropic response to clinically relevant concentrations of ethanol (18 to 71 mmol/L) using papillary muscles from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) at 10 and 25 weeks of age. Mechanical parameters measured were peak tension developed, time to peak tension, time to 90% relaxation, and maximal velocities of tension development and tension decline. SHR exhibited elevated systolic pressure and body weight as well as cardiomegaly and hepatomegaly at 10 and 25 weeks of age. Baseline mechanical properties were similar in SHR and WKY muscles at 10 weeks, whereas at 25 weeks, SHR muscles developed less tension, and both maximal velocities of tension development and tension decline were markedly depressed. Ethanol exposure produced concentration-dependent negative inotropic effects in both groups at both ages. Ethanol (> 18 nmol/L) decreased peak tension developed in both groups at 10 weeks, although higher concentrations were required at 25 weeks. The negative inotropic effect of ethanol resulted in the shortening of time to 90% relaxation in both groups at 10 weeks and was associated with a slowing of maximal velocities of both tension development and tension decline. The results suggest that aging depresses baseline mechanical properties when coupled with hypertension. In addition, the magnitude of the negative inotropic effect of ethanol was attenuated in both groups at 25 weeks of age.

Acute and chronic effects of ethanol on papillary muscles from spontaneously hypertensive rats.

The effects of chronic ethanol ingestion (12 weeks) on the mechanical properties of hypertrophied papillary muscle and the in vitro effects of ethanol (80-640 mg/dl) was studied. Papillary muscles from spontaneously hypertensive rats (SHRs) and their normotensive controls, the Wistar-Kyoto rat (WKY), were used in this study. Peak-developed tension was significantly less in muscles obtained from SHR compared with WKY even when normalized for muscle cross-sectional area. Chronic ethanol ingestion resulted in a significant shortening of both contraction and relaxation duration in muscles from SHR and WKY. In muscles from SHR and WKY, acute in vitro ethanol exposure produced concentration-dependent negative inotropic effects that were associated with a reduction in the duration of contraction and relaxation and marked slowing in the maximum velocities of tension development and decay. These findings suggest that the contractile response to ethanol exposure, in vitro, is not modified by either chronic ethanol ingestion or hypertension.

Influence of calcium channel blocker treatment on the mechanical properties of diabetic rat myocardium.

The objective of this investigation was to determine whether calcium channel blocker (CCB) treatment effectively restores normal baseline mechanical function in diabetic myocardium and to evaluate its effect on the interval-strength relationship. Wistar rats were made diabetic with streptozotocin (55 mg/kg, IV). Left-ventricular papillary muscles from normal and diabetic (10 weeks) rats were superfused with Tyrode's solution at 30 degrees C. A subgroup of diabetic and normal animals received daily injections of verapamil or nifedipine (10 mg/kg, IP; 8 weeks) to compare the effectiveness of a phenylalkylamine to a dihydropyridine in reversing diabetes-induced contractile dysfunction in vitro. Muscles were electrically stimulated at 0.5 Hz with suprathreshold stimuli, and the following parameters were measured: peak tension developed, time to-peak tension, time-to-90% relaxation, and the maximum velocities of tension development and decay. Experimental diabetes was characterized by: severe hyperglycemia, hepatomegaly, reduced body weight gain, cardiomegaly, and increased plasma phospholipid levels. In addition, baseline values of peak tension developed, time to-peak tension, and time-to-90% relaxation were significantly greater in muscles from diabetic animals. Chronic nifedipine treatment reduced hyperglycemia and plasma phospholipid levels, normalized body weight gain, and reduced both heart and liver sizes in diabetic animals. Nifedipine treatment completely reversed diabetes-induced prolongation in both time-to-peak tension and time-to-90% relaxation. In diabetic myocardium, a slightly positive component was present in the interval-strength relationship between 0.01 and 1 Hz, resulting in a rightward shift in the entire curve across a wide range of stimulation frequencies (0.01-5 Hz). This positive component was absent in muscles from diabetic animals treated with both CCBs, and verapamil produced a leftward shift in the frequency response curve. The results of this study suggest that chronic nifedipine treatment may be more effective than verapamil in restoring normal baseline myocardial mechanical function, reducing hyperglycemia and hyperlipidemia, as well as attenuating both cardiac and liver enlargement in experimental diabetes. In contrast, verapamil treatment tended to normalize more effectively the inotropic response to changes in stimulation frequency in diabetic myocardium.

Differential effects of chronic calcium channel blocker treatment on the inotropic response of diabetic rat myocardium to acute ethanol exposure.

Cardiomyopathy is a consistent feature of diabetic myocardium as well as in prolonged alcohol consumption. Diabetes-induced myocardial dysfunction has been attributed, in part, to calcium overload within individual myocytes. The present study compares the effectiveness of the calcium channel blocker nifedipine (dihydropyridine-type) with verapamil (phenylalkylamine-type) in reversing myocardial dysfunction and diminishing the negative inotropic effect of ethanol on diabetic rat myocardium. Wistar rats were made diabetic with streptozotocin (55 mg/kg, i.v.) and isolated electrically stimulated papillary muscles were studied under isometric conditions in the absence and presence of clinically relevant concentrations of ethanol (80-240 mg/dl, i e., 17.4-52.1 mM). Subgroups of diabetic and normal animals received daily injections of verapamil or nifedipine 2 weeks after induction of diabetes for 8 weeks. Untreated diabetic animals exhibited hyperglycemia, hyperlipidemia, reduced growth, cardiomegaly, and hepatomegaly. Compared to verapamil chronic nifedipine treatment normalized or reversed the effects of diabetes on myocardial mechanical function. The negative inotropic effect of ethanol was attenuated only in muscles from verapamil-treated diabetic animals. Thus, chronic nifedipine treatment may be more effective than verapamil in reducing hyperglycemia, attenuating both cardiac and liver enlargement, and restoring myocardial mechanical function, in experimental diabetes. However, chronic verapamil therapy is more effective in diminishing the negative inotropic effect of ethanol on diabetic myocardium. These findings may have clinical significance among diabetic patients who consume alcoholic beverages while receiving long-term calcium blocker therapy.

Influence of insulin treatment on the mechanical properties and inotropic response to ethanol in diabetic myocardium.

Diabetes significantly affects cardiac performance, as does ethanol. To assess whether insulin treatment alters the inotropic response to acute ethanol exposure and reverses diabetes-induced myocardial dysfunction, male Wistar rats were made diabetic using streptozocin, 55 mg/kg intravenously. The inotropic effect of ethanol on normal, diabetic, and insulin-treated (8 weeks) diabetic animals was studied using isolated, left-ventricular papillary muscle preparations superfused with Tyrode's solution under isometric conditions. Peak tension developed, time to peak tension (TPT), time to 90% relaxation (RT90), maximum rate of tension developed (+VT), and maximum rate of fall in tension (-VT) were determined in the absence and presence of clinically relevant concentrations of ethanol for 10 minutes. In insulin-treated diabetic muscles, baseline developed tension, +VT, and -VT were enhanced, and the prolongation of TPT and RT90, characteristic of diabetic myocardium, was attenuated. The magnitude of the reduction in developed tension in response to ethanol, 80 mg/dL, was slightly greater in untreated diabetic myocardium. Higher concentrations of ethanol (120 to 240 mg/dL) decreased tension in all groups and was of similar magnitude. The negative inotropic effect of higher ethanol concentrations was associated with shortening of TPT and RT90, as well as a diminution of +VT and -VT. It is concluded that with insulin treatment, the mechanical properties of diabetic myocardium are normalized; however, neither the myocardium's sensitivity to ethanol nor the overall magnitude of ethanol's negative inotropic effect is modified by insulin treatment.

Effects of acute acetaldehyde, chronic ethanol, and pargyline treatment on agonist responses of the rat aorta.

The purpose of this investigation was to determine the mechanism(s) underlying the vasorelaxant effects of acute versus chronic acetaldehyde (ACA) exposure, in particular, the role of intact endothelium on contractile responses of rat aortic ring segments to potassium chloride (KCl) or norepinephrine (NE). The acute effects of ACA were assessed in preparations from normal animals maintained on a standard rat chow (non-ethanol-ingesting). The monoamine oxidase inhibitor, pargyline, elevates plasma ACA levels by decreasing acetaldehyde dehydrogenase activity. Accordingly, preparations from pargyline-treated ethanol-ingesting animals (PE) were used to assess the effects of chronic (12 weeks) ACA and results were compared to those of pargyline-treated non-ethanol-ingesting (P) rats fed a standard liquid diet. In normal and P preparations, maximal inotropic response to KCl and NE were greater in endothelium-denuded than in endothelium-intact preparations. However, in aortic rings obtained from PE rats, the maximal inotropic response to KCl was depressed only in endothelium-denuded rings and that to NE was nearly identical in endothelium-denuded compared to endothelium-intact preparations. Acute ACA (30 mM) exposure significantly reduced both NE- and KCl-induced contractile responses in muscles from all groups. The magnitude of the vasorelaxant effect of this [ACA] on NE-induced responses was endothelium-independent and was similar between groups. However, the vasorelaxant effect of ACA (30 mM) on KCl-induced contractile responses was significantly attenuated in muscles from PE animals with greater inhibition occurring in endothelium-denuded preparations. These results suggest that chronic acetaldehyde exposure leads to an impairment in the inotropic response to membrane depolarization in endothelium-denuded preparations resulting in depressed responsiveness. In addition, the acute vasorelaxant effect of ACA on KCl-induced contractures is significantly attenuated in preparations chronically exposed to ACA which suggests a possible development of tolerance.

Effects of acetaldehyde on the isolated papillary muscle of diabetic rats.

The effects of acetaldehyde (ACA) were examined in isolated electrically driven papillary muscle preparations from normal and streptozotocin-treated diabetic rats. Muscles from diabetic rats developed greater tension than those from normal rats. In muscles from both groups, ACA caused concentration-dependent negative inotropic effects that were independent of cholinergic or purinergic mechanisms and were not attributable to nitric oxide (NO) release. ACA was three to five times more potent with regard to its negative inotropic effect in diabetic than in normal rat muscles. A propranolol-sensitive, sympathetically mediated positive inotropic effect occurred at certain concentrations. Decreasing [Ca2+]o from 2.7 to 0.5 mM reduced basal developed force to a significantly greater extent in muscles from normal rats than in those from diabetic rats. In low [Ca2+]o, concentration-response curves to CaCl2 in diabetic muscles were displaced to the left of that in normal muscles, suggesting that diabetic muscles are more sensitive to the positive inotropic effect of added CaCl2 at low [Ca2+]o, whereas at higher [Ca2+]o (> 1 mM), normal muscles developed more force in response to added CaCl2. ACA 10 and 30 mM more readily inhibited CaCl2-induced positive inotropic effect in normal than in diabetic muscles. Force-frequency curves, (negative staircase response) were recorded in both normal and diabetic muscles. In diabetic muscles, the curve exhibited a positive component at the lowest frequencies applied and was displaced to the right of that in normal muscle. ACA concentration-dependently inhibited force development, and diabetic muscles were more susceptible to the negative inotropic effect of ACA, when the stimulation frequency was increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of ethanol and quinidine on contractility and myocyte action potential in the rat ventricle.

The combined effects of ethanol and quinidine on cardiac electromechanical coupling are unknown, but both drugs affect cardiac conduction and can cause myocardial depression. Isolated left ventricular papillary and ventricular myocytes were used to assess the combined effects of quinidine and ethanol on the electrophysiologic and mechanical properties of rat myocardium. The combination of quinidine (1-300 microM) and ethanol (120-240 mg/dL) depressed active papillary muscle tension within the clinically useful concentration range. In electrophysiologic studies of isolated ventricular myocytes, quinidine prolonged the action potential duration at 50% (APD50) and 90% (APD90) repolarization, the absolute refractory period, and the relative refractory period, but decreased the maximum rate of change of depolarization in phase 0 (Vmax). When cells were exposed to ethanol (240 mg/dL) and quinidine (1.5 microM) together, a significant decrease in the quinidine-induced prolongation of the absolute refractory and relative refractory periods was seen. Additional changes in action potential parameters from the quinidine values included slight reductions in Vmax and in APD50 and APD90, but these reductions were not consistently displayed, nor were they statistically significant.

Chronic verapamil treatment attenuates the negative inotropic effect of ethanol in diabetic rat myocardium.

It is well established that cardiomyopathy is a consistent feature of diabetic myocardium and that alcohol consumption increases the risk of cardiovascular disease among diabetic subjects. The objective of this investigation was to determine whether acute or chronic verapamil treatment attenuates the negative inotropic effect of ethanol (EtOH) in the diabetic rat heart. Wistar rats were made diabetic with streptozotocin (55 mg/kg, iv). Left-ventricular papillary muscles, from normal and diabetic (8 weeks) rats, were superfused with Tyrode's solution at 30 degrees C while driven at 0.5 Hz. A subgroup of diabetic and normal animals received daily injections of verapamil (8 mg/kg, ip; 8 weeks), whereas muscles from untreated animals were exposed to verapamil (2 microM) in vitro. Peak tension developed (PTD), time to peak tension (TPT), time to 90% relaxation (RT90), and the maximum velocities of tension development (+VT) and decay (-VT) were determined in the absence and presence of clinically relevant concentrations of EtOH (80-240 mg/dL, i.e., 17.4-52.1 mM). Ethanol at 80 mg/dL reduced PTD, +VT, and -VT only in preparations from diabetic animals. Higher concentrations of EtOH (120-240 mg/dL) decreased PTD, TPT, +VT, and -VT. The negative inotropic effect of EtOH (240 mg/dL) was attenuated only in diabetic myocardium chronically treated with verapamil, whereas acute verapamil treatment potentiated the negative inotropic effect of EtOH in both normal and diabetic myocardium. Thus, chronic verapamil therapy diminishes the negative inotropic effect of EtOH in diabetic myocardium and acute verapamil treatment exaggerates it.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of electrical stimulation, adenosine and adenosine-5'-triphosphate (ATP) on mouse rectal muscle.

The effects of electrical field stimulation and of purine compounds, adenosine and adenosine-5'-triphosphate (ATP) were examined on the mouse isolated rectum. Electrical field stimulation induced frequency-dependent contractions of mouse rectal muscles which were potentiated by physostigmine and inhibited by atropine or tetrodotoxin. Contractile amplitude at 37 degrees C was significantly (P less than 0.05) greater than at 25 degrees C, but the degree of potentiation by physostigmine was significantly (P less than 0.05) greater at 25 degrees C. ATP (1.6 x 10(-4)-1.28 x 10(-3) M) and adenosine (1.8 x 10(-4)-1.48 x 10(-3) M) inhibited in concentration-related fashion contractile responses induced by KCl (1.34 x 10(-2) M-1.07 x 10(-1) M) by acetylcholine (2.2 x 10(-7) M-1.4 x 10(-5) M) and by CaCl2 in high KCl (120 mM)-CaCl2-free Tyrode solution. Theophylline and quinidine ('purinoceptor' antagonists) antagonized ACH contractile effects and so could not be satisfactorily employed in the characterization of the purine receptors in the mouse rectum. It may be concluded from this study that in the mouse rectum, acetylcholine is an excitatory neurotransmitter and that there is a non-adrenergic, non-cholinergic inhibitory neuromuscular transmission in this tissue. Further, ATP and adenosine have been demonstrated to cause relaxation in this tissue by possibly a post-synaptic mechanism involving inhibition of Ca2+ influx into the depolarized muscle.

A comparison of the actions of 4-aminopyridine, caffeine and quinine on the toad isolated rectus abdominis muscle.

1. 4-Aminopyridine (4-AP)-induced contractures have been compared with those evoked by caffeine and quinine on the toad rectus abdominis muscle. 2. All three compounds produced slowly-developing sustained contractures. The time to half maximal contracture and relaxation was significantly longer for 4-AP than for caffeine or quinine. 3. Verapamil and manganese inhibited 4-AP, caffeine and quinine-induced contractures. 4. Ca2+-free-EGTA Ringer and procaine severely inhibited caffeine and quinine responses, but 4-AP contractures were relatively unaffected. 5. In depolarizing (100 mM K+) Ringer solution, caffeine and quinine responses were reduced to 6-9% of their controls. 4-AP responses were reduced by about 25%. 6. It is concluded that in the toad rectus muscle, 4-AP-induced contractures differ from those produced by caffeine and quinine, and appear to rely mainly on the release of intracellular located Ca2+, while caffeine and quinine are considered to act predominantly on plasma membrane sites.

Caffeine- and potassium-induced contractures of mouse isolated soleus muscle: effects of verapamil, manganese, EGTA and calcium withdrawal.

1. The effects of verapamil, manganese, EGTA and Ca2+-withdrawal on caffeine and potassium-induced contractures of the mouse isolated soleus muscle have been studied. All four treatments profoundly inhibited, in concentration-dependent manner, both K+- and caffeine-induced contractures, and recovery from these treatments was slight. 2. Caffeine (5.14-51.4 mmol/l), elicited biphasic contractures, characterized by an initial phasic, and subsequent tonic contractures. K+-induced contractures were monophasic except at the higher concentrations (53.4-214 mmol/l), when biphasic responses were also recorded. The biphasic K+ contracture had a time course similar to the caffeine-induced response. 3. Both phases of the caffeine and K+ responses were abolished by the introduction of CA2+-free Krebs'-Henseleit solution (KHS). Increasing [Ca2+]0 from 1 to 5 mmol/l, markedly inhibited the amplitude of caffeine and K+ contractures. 4. In high [K+]0 KHS, peak contractures to caffeine were reduced to about 50% of those in normal KHS. In the presence of 15.4 mmol/l caffeine, the responses to lower concentrations of K+ (13.4-26.8 mmol/l) increased, while responses evoked by the higher K+ concentrations were depressed compared with controls in normal KHS. 5. Pancuronium or D-tubocurarine severely inhibited caffeine-induced contractures, but only slightly inhibited K+-induced responses. 6. It is suggested that both caffeine- and K+-induced contractures of the mouse soleus are dependent upon the presence of [Ca2+]0, and that these contractures possibly occurred as a result of extracellular Ca2+-influx.

Differential blockade of potassium and carbachol contractures of toad isolated rectus abdominis muscle by calcium entry blockers, ketamine and hydrallazine.

Contractures of the rectus abdominis muscle of Buffo regularis evoked by carbachol were inhibited in concentration-dependent fashion by verapamil, nifedipine, hydrallazine and ketamine, whereas KCI-induced contractures were relatively resistant to blockade by these compounds. The order of potency in blocking carbachol-induced responses was: verapamil greater than nifedipine greater than hydrallazine greater than ketamine, verapamil being over 10 times more potent than nifedipine. In high K+-depolarized muscles, carbachol elicited contractural responses which were inhibited by verapamil, nifedipine, hydrallazine and ketamine; these compounds exhibited similar blocking potencies against carbachol responses in high K+-depolarized muscles as in the polarized muscles. In Ca2+-free EGTA-Ringer's solution, both KCl and carbachol contractures were abolished, but were restored on readmission of Ca2+. d-Tubocurarine competitively antagonized carbachol, but not KCl contractures. It is suggested that (i) extracellular Ca2+-influx occurs during contractures of the toad rectus abdominis muscle induced by carbachol and KCl; (ii) there probably exists more than one pathway for Ca2+-entry into this muscle--one pathway is stimulated by high K+-depolarization, and another which is linked to the activation of nicotinic cholinoceptors. Verapamil, nifedipine, hydrallazine and ketamine appear to preferentially inhibit Ca2+-influx stimulated via a pathway linked to nicotinic cholinoceptors.

Influence of calcium lack and nifedipine on carbachol-, potassium- and quinine-induced contractions of the lizard isolated rectum.

In the lizard isolated rectum, carbachol-, KCl- and quinine-induced contractions were reversibly inhibited by calcium withdrawal. Quinine-induced contractions were more sensitive to calcium lack than were those elicited by carbachol or KCl. Nifedipine inhibited contractile responses induced by all three agonists, but more readily blocked quinine- and carbachol- than KCl-induced contractile responses. It is suggested that extracellular calcium influx stimulated by carbachol, KCl and quinine occurs via different pathways, and that differentiation of calcium channels exists in lizard smooth muscle.

Actions of quinine on the rat isolated rectum.

The actions of quinine on the rat isolated rectum was studied. Quinine (2.5 X 10(-5) - 2.5 X 10(-4) M) initiated rhythmic phasic contractions (RPC) which were prevented by verapamil, lanthanum, indomethacin or Ca2+-withdrawal, but were unaffected by tetrodotoxin, L-propranolol, phentolamine, or atropine. Higher quinine concentrations (2.5 X 10(-4) - 10(-3) M) relaxed the rat rectum and inhibited the RPC induced by lower concentrations. Exposure to high Ca2+-Tyrode solution (6 Ca2+-Tyrode) increased the baseline tone of the muscle. Under this condition, quinine (2.5 X 10(-4) - 10(-3) M), but not lower concentrations relaxed the muscle. Quinine (2.5 X 10(-5) - 10(-3) M) concentration-dependently inhibited KCl and acetylcholine-induced contractions, but more readily blocked KCl than acetylcholine-induced responses; the inhibitory effect of quinine against KCl, but not acetylcholine, was largely reversed by increasing the Ca2+ concentration of the Tyrode solution. It is concluded that quinine-induced RPC occurred independently of adrenergic or cholinergic mechanisms, and are likely to be due to enhanced Ca2+ influx into the rectum, while the relaxation encountered with higher concentrations may be due to inhibition by quinine of transmembranal Ca2+ influx. Evidence was obtained that quinine may preferentially inhibit Ca2+ influx through a pathway linked to high K+ depolarisation.

A comparison of the effects of 4-dimethylaminopyridine and 4-aminopyridine on isolated cardiac and skeletal muscle preparations.

The actions of 4-dimethylaminopyridine (4-dimethyl-AP) on the isolated rat diaphragm and guinea-pig atria were examined and compared with those of 4-aminopyridine (4-AP). In the rat diaphragm, 4-dimethyl-AP slightly (27%) increased indirectly evoked maximal twitches, and was a weak antagonist of d-tubocurarine-induced neuromuscular block, while 4-AP increased indirect twitches by about 250%, and produced rapid and complete reversal of d-tubocurarine block. In directly stimulated preparations, 4-AP but not 4-dimethyl-AP produced marked augmentation of maximal twitches (220%). In electrically-driven guinea-pig left atria, both compounds produced positive inotropic effects (PIE), which were partially inhibited by L-propranolol or by pretreatment with intraperitoneal syrosyngopine or 6-hydroxydopamine, suggesting that part of the response was due to noradrenaline (NA) release. Nifedipine or methoxyverapamil (D-600) produced negative inotropic effects (NIE) and prevented the PIE of both aminopyridines. The NIE of these Ca2+ entry blockers was reversed by excess CaCl2 or NA. It is concluded that 4-dimethyl-AP, like 4-AP is a powerful cardiac inotropic agent in vitro. 4-Dimethyl-AP which, unlike 4-AP, possesses weak effects on neuromuscular transmission and on muscle contractility in the rat diaphragm, therefore appears to possess a greater specificity for cardiac muscle.

Effect of adenosine 5'triphosphate (ATP) on the isolated rectum of the rainbow lizard Agama agama.

ATP but not adenosine contracts lizard isolated rectal muscles. Theophylline, quinidine, nifedipine and lack of Ca2+ inhibited ATP contractions, but dipyridamole, indomethacin, atropine, D-tubocurarine, phentolamine, L-propranolol, cimetidine, mepyramine and methysergide did not significantly modify the contractions. ATP inhibited Ca2+-induced contractions in Na+-free, high K+ media. In normal Ringer, it relaxed carbachol-contracted muscles and inhibited KCl contractions. The results suggest that ATP contractions are dependent upon external Ca2+, and are probably associated with extracellular Ca+ influx. The contractions appear to have occurred independently of receptor activation and may have been mediated by a depolarising action of the nucleotide. The relaxant effect of ATP possibly occurred as a consequence of inhibition of Ca2+ influx into carbachol or K+-depolarised muscles.

Contractile effects of 4-aminopyridine on isolated frog rectus abdominis muscles.

The contractile effects of 4-aminopyridine (4-AP) on isolated frog rectus abdominis muscles were examined, and compared with KCl-induced contractures. 4-AP (1-40 mM) caused slowly developing, concentration-dependent contractures which were not modified by (+)-tubocurarine (2.7-13.3 microM). The contractures were prolonged and very slowly relaxed (greater than 30 min) on washout. KCl-induced contractures developed more rapidly and relaxation was equally rapid, both occurring within 90 s of application and washout, respectively. KCl contractures were slightly but significantly (P less than 0.05) attenuated by (+)-tubocurarine in concentrations that blocked carbachol contractures. In calcium-free Ringer's solution, KCl (10-120 mM) responses were completely abolished, but 4-AP concentration-response curves were shifted to the right three- to four-fold. The results show that 4-AP causes contracture of the frog rectus abdominis. It is suggested that at the lower concentration employed (less than 10 mM), 4-AP increases extracellular calcium entry into the muscle, while larger concentrations produce contractures by a direct intracellular mechanism. 4-AP contractures were independent of postjunctional nicotinic cholinoceptor activation.

The effect of 4-aminopyridine on the frequency-force relationship of isolated left atria of guinea-pigs.

The effect of 4-aminopyridine (4-AP) on the frequency-force relationship of isolated electrically-driven left atria of guinea-pigs was studied. 4-AP increased atrial contractility between the optimum and pessimum frequencies, but not the rested state contractions. The compound increased the time to peak force and the relaxation time. Results obtained indicate that 4-AP-induced positive inotropic effect is different from that induced by noradrenaline, and supports the view that the compound exerts a direct action on the myocardium to increase contractility. Release of endogenous noradrenaline by 4-AP cannot, however, be excluded.