The Global Leadership Mentoring Community: building capacity across seven global regions.

AIM: The purpose of this paper is to report on the evaluation of the online Global Leadership Mentoring Community, a programme designed to build relationships across seven global regions and promote leadership development for emerging nurse leaders. BACKGROUND: There is a pressing need and opportunity for sustainable global leadership mentoring programmes. This programme of Sigma Theta Tau International (Sigma) brought mentors and mentees together from across the world to build leadership capacity, understand global leadership issues and build networks. Community coordinators purposively selected mentors from each of Sigma's seven Global Regions, and mentees were chosen through a process of snowball sampling. Mentors and mentees met monthly with quarterly group calls. METHODS: The study followed a programme evaluation, outcomes-focused approach. All eleven pairs of mentors-mentees were invited to complete online surveys at the outset and end of programme capturing both quantitative and qualitative data. Quantitative data were analysed using descriptive statistics and for qualitative data, a thematic analysis. FINDINGS: Quantitative data confirmed that all 22 participants gained from the experience. From qualitative analysis, themes emerged illustrating the scope of achievements: 1. facilitation of successful outcomes for both mentors and mentees, 2. challenges of global mentoring and 3. strategies for successful global mentoring. DISCUSSION/CONCLUSION: Participants reported that creating global leadership is a longitudinal process that needs sustained attention to effect change. This evaluation identified many strengths of the programme and recommended its continuation to help further development of global leaders, particularly through focusing more purposefully on policy issues. IMPLICATIONS FOR NURSING POLICY: Empowerment of nurses globally through a Global Leadership Mentoring Community can improve leadership at all levels, thus emboldening their voices to influence nursing and health policy and ultimately improve patient care.

Cost-consequence analysis comparing 2-octyl cyanoacrylate tissue adhesive and suture for closure of simple lacerations: a randomized controlled trial.

STUDY OBJECTIVE: We investigate the cost difference between conventional suture and tissue adhesive methods in simple wound closure. METHODS: A cost-consequence analysis was conducted alongside a nonblinded randomized controlled trial comparing 2-octyl cyanoacrylate tissue adhesive with conventional suture in simple lacerations closure in emergency departments (EDs) of a university teaching hospital and a major regional hospital in Hong Kong. One hundred eighty-six adult patients with simple lacerations of length within 8 cm were randomized to receive tissue adhesive (93 patients) or conventional suture (93 patients) for wound closure. The primary outcome measures were the costs to the Hospital Authority and the charges on participants incurred in each treatment method. The secondary outcome measures included the cosmetic visual analog scale, visual analog scale, Wound Evaluation Score, total time spent in each closure method, and the overall patients' satisfaction on the whole process of wound management. RESULTS: The 2 groups had similar baseline characteristics. The tissue adhesive method incurred a higher cost to the Hospital Authority (216.12 [US $27.70] versus 171.33 [US $21.96]; absolute difference 44.79 [US $5.74] [95% confidence interval (CI) 32.76 to 55.95 [US $4.20 to 7.14]]) but a lower charge to patients (109.68 [US $14.06] versus 156.96 [US $20.12]; absolute difference 47.28 [US $6.06] [95% CI, 35.58 to 58.98 [US $4.56 to 7.56]) than the conventional suture method. The mean cosmetic visual analog scale score, visual analog scale score, and Wound Evaluation Score of the 2 groups were similar at various intervals within 3 months after wound closure. Compared with the suture group, the tissue adhesive group had a shorter median procedure time, fewer patients had wound erythema or swelling after wound closure, fewer patients required analgesics on discharge at ED, and there was a higher overall patient satisfaction score. CONCLUSION: Simple wounds closed by tissue adhesives incur a higher cost to the Hospital Authority than the conventional suture but may be favored by patients because of lower personal charge.

Prevalence of workplace violence against nurses in Hong Kong.

OBJECTIVES: To determine the prevalence and nature of workplace violence against nurses, and how nurses deal with such aggression; and to identify the risk factors related to violence in the hospital environment. DESIGN: Cross-sectional questionnaire study. SETTING: University teaching hospital, Hong Kong. PARTICIPANTS: All nursing staff in the hospital, except nurses who were unable to read Chinese or who did not have patient contact (eg those worked in administrative positions), were invited to complete a questionnaire. MAIN OUTCOME MEASURES: Demographic data of the respondents, incidence of and risk factors contributing to workplace violence. RESULTS: A total of 420 nurses returned the completed questionnaire (response rate, 25%). Three hundred and twenty (76%; 95% confidence interval, 72-80%) nurses reported abuse of any kind--verbal abuse, 73%; bullying, 45%; physical abuse, 18%; and sexual harassment, 12%. Most (82%) nurses who experienced verbal abuse tended to confide in friends, family members, or colleagues. Some (42%) ignored the incident. Risk factors for workplace violence included: working in male wards and in certain specialties such as the Accident and Emergency Department, Community Nursing Service, and the Orthopaedics and Traumatology Department. CONCLUSION: Workplace violence against nurses is a significant problem in Hong Kong. Further large-scale studies should be conducted to more closely examine the problem.

Developmental changes in regulation of the Na+, K(+)-ATPase alpha 3 isoform by thyroid hormone in ferret heart.

Ferret heart expresses the alpha 1- as well as the alpha 3-isoform of the Na+, K(+)-ATPase. We have shown previously that the alpha 3 isoform is differentially upregulated during postnatal cardiac development and that in adult ferrets expression of alpha 3 is not responsive to regulation by thyroid hormone (TH). Since developmental-stage dependent effects of TH have been reported previously, the present study examined whether effects of TH on expression of the Na+, K(+)-ATPase isoforms in ferret heart is modulated during development and possible mechanisms were examined. Ferrets of different age groups were treated with TH and the relative abundance of Na+, K(+)-ATPase isoforms in ferret myocardium was determined by immunoblotting. Thyroid hormone (T3; 50 micrograms/100 g body weight on 3 alternating days, s.c.) increased protein levels of the alpha 3 isoform, but not that of alpha 1 or beta 1, in myocardium of 5-day-old and 3-week-old ferrets. By contrast, in myocardium of 6- and 8-week-old ferrets T3 failed to increase protein levels of alpha 1 and alpha 3. To determine whether elevated plasma levels of TH during development plays a role in the transition, mature ferrets were first made hypothyroid before TH treatment. In these hypothyroid ferrets expression of the alpha 3 isoform remained unresponsive to TH (T4, 0.5 mg/kg for 7 days, s.c.). The transition from TH-responsive to TH-unresponsive appears to be isoform-specific because in skeletal muscle of 8-week-old ferrets and in hypothyroid ferrets the alpha 2 isoform is upregulated by TH. Finally, there appears to be functional thyroid hormone receptors throughout development because in each age group TH effectively induced expression of alpha-MHC in the myocardium. In conclusion, these findings demonstrate that expression of alpha 3 isoform in the myocardium of newborn ferret is responsive to TH; however, the responsiveness terminates between 3- and 6-weeks of age. Neither elevated endogenous TH level nor a lack of functional thyroid hormone receptor appears to be responsible for the transition from TH-responsive to TH-unresponsive.

Productivity losses associated with diabetes in the US.

OBJECTIVE: The objective of this study was to estimate the cost of productivity losses in the U.S. attributable to diabetes, with regard to specific demographic and disease-related characteristics in the U.S.. RESEARCH DESIGN AND METHODS: We used the 1989 National Health Interview Survey, a random survey of individuals in the U.S. that included a diabetes supplement. Data on individuals were obtained for labor force participation, hours of work, demographic and occupational characteristics, self-reported health status, and several variables that indicated the presence, duration, and severity (complications) of diabetes. Using multivariate regression analyses, we estimated the association of independent variables (e.g., demographics, health, and diabetes status) with labor force participation, hours of work lost, and the economic value of lost work attributable to diabetes and its complications and duration. RESULTS: In general, the presence of diabetes and complications were found to be related to workforce participation variables. The magnitude of the lost-productivity costs depended on personal characteristics and on the presence and status of diabetes. In general, the loss of yearly earnings amounted to about a one-third reduction in earnings and ranged from $3,700 to $8,700 per annum. CONCLUSIONS: Diabetes has a considerable net effect on earnings, and the complications and duration of diabetes have compound effects. Our findings have implications for the cost-effectiveness of diabetes control; the presence of complicating factors is the single most important predictive factor in lost productivity costs attributable to diabetes, and thus the avoidance or retardation of complications will have an impact on indirect health-related costs.

Effects of norepinephrine on expression of IGF-1/IGF-1R and SERCA2 in rat heart.

OBJECTIVES: The effects of norepinephrine on expression of cardiac genes during pathological cardiac growth and heart failure are not fully understood. Tissue insulin-like growth factor 1 (IGF-1) and its receptor (IGF-1R) play an important role in the regulation of the hyperplastic capacity of cardiac myocytes. Sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2), on the other hand, is important in regulating cardiac contractile function. The present study examined the effects of elevated levels of NE on expression of IGF-1/IGF-1R and SERCA2 mRNAs. METHODS: Rats were infused with NE using osmotic minipumps for 3 and 6 days at a rate of 50 micrograms/kg/h and also at a higher dose (130 micrograms/kg/h) for 6 and 14 days. Levels of expression of IGF-1/IGF-1R and SERCA2 mRNAs were determined by ribonuclease protection assay and by Northern blotting, respectively. RESULTS: NE treatment significantly increased IGF-1 mRNA levels in both left- and right-ventricle; however, levels of IGF-1R increased in the left- but not the right-ventricle. By contrast, NE infusion at both the lower dose and the higher dose failed to alter expression of SERCA2 mRNA. CONCLUSION: Our results suggest that NE treatment differentially regulates expression of IGF-1 and IGF-1R in the ventricles of rat heart and that NE appears not to affect expression of SERCA2 mRNA.

Okadaic acid stimulates ouabain-sensitive 86Rb(+)-uptake and phosphorylation of the Na+/K(+)-ATPase alpha-subunit in rat hepatocytes.

Ca(2+)-mobilizing and cAMP-dependent hormones rapidly increase sodium, potassium-dependent adenosine triphosphatase (Na+/K(+)-ATPase)-mediated transport in rat hepatocytes. To explore the possible role of protein phosphatases in these responses we used a protein phosphatase inhibitor, okadaic acid. Okadaic acid stimulation of ouabain-sensitive 86Rb(+)-uptake was maximal between two and three minutes and displayed an EC50 of 41 +/- 1 nM. Inhibition of Na+/H+ exchange with an amiloride analog abolished the response to insulin, but had no effect on okadaic acid-mediated stimulation of Na+/K(+)-ATPase transport. In hepatocytes metabolically-radiolabeled with 32Pi, okadaic acid stimulated the incorporation of radioactivity into several 95 kDa peptides, one of which reacted with anti-LEAVE peptide antisera, that recognizes Na+/K(+)-ATPase alpha-subunits. In other experiments Na+/K(+)-ATPase was immunoprecipitated from detergent-solubilized membrane fractions of metabolically-radiolabeled cells with an antisera to purified rat kidney Na+/K(+)-ATPase. A 95 kDa phosphoprotein was immunoprecipitated using anti-Na+/K(+)-ATPase antisera, but not by preimmune serum. Okadaic acid stimulated incorporation of radioactivity into this band by 220 +/- 28%. These findings provide support for the hypothesis that rapid stimulation of hepatic Na+/K(+)-ATPase by hormones may be related to protein kinase/phosphatase-mediated changes in the phosphorylation state of the Na+/K(+)-ATPase alpha-subunit.

Ascorbic acid: an endogenous inhibitor of isolated Na+,K+-ATPase.

During attempts to isolate and identify an endogenous ligand for the glycoside binding sites on Na+,K+-ATPase, bovine adrenal glands were found to contain a potent inhibitor of isolated Na+,K+-ATPase. The inhibitory principle was extracted from adrenal cortex, following homogenization in NaHCO3 solution and separation on a Sephadex G-10 column. The active principle was recovered from a fraction which eluted from the column after the 3H2O peak. The extract inhibited isolated Na+,K+-ATPase and the specific [3H]ouabain binding reaction. Sensitivity of the enzyme to the inhibitory action of the extract was species and tissue dependent; however, the pattern and the magnitude of the sensitivity were different from those of the digitalis glycosides. Moreover, the inhibitory principle failed to inhibit sodium pump activity, estimated from ouabain inhibitable 86Rb+ uptake by guinea pig brain slices. The activity of the extract to inhibit isolated Na+,K+-ATPase was stable under acidic condition but was lost rapidly at neutral pH, and could be eliminated by EDTA. In an acidic medium, the inhibitory principle had an absorption maximum at 244 nm which shifted to 264 nm and decayed rapidly at neutral pH. By using mass spectrometry, the principle was identified to be ascorbic acid, which has been shown previously to inhibit isolated Na+,K+-ATPase under appropriate conditions. Because ascorbic acid was incapable of inhibiting the sodium pump in intact cells, this inhibitor of the isolated enzyme does not appear to be the endogenous ligand which regulates sodium pump activity in vivo.

Age-associated differential expression of Na(+)-K(+)-ATPase subunit isoforms in skeletal muscles of F-344/BN rats.

Skeletal muscle expresses multiple isoforms of the Na(+)-K(+)-ATPase. Their expression has been shown to be differentially regulated under pathophysiological conditions. In addition, previous studies suggest possible age-dependent alterations in Na(+)-K(+) pump function. The present study tests the hypothesis that advancing age is associated with altered Na(+)-K(+)-ATPase enzyme activity and isoform-specific changes in expression of the enzyme subunits. Red and white gastrocnemius (Gast) as well as soleus muscles of male Fischer 344/Brown Norway (F-344/BN) rats at 6, 18, and 30 mo of age were examined. Na(+)-K(+)-ATPase activity, measured by K(+)-stimulated 3-O-methylfluorescein phosphatase activity, increased by approximately 50% in a mixed Gast homogenate from 30-mo-old compared with 6- and 18-mo-old rats. Advancing age was associated with markedly increased alpha(1)- and beta(1)-subunit, and decreased alpha(2)- and beta(2)-subunit in red and white Gast. In soleus, there were similar changes in expression of alpha(1)- and alpha(2)-subunits, but levels of beta(1)-subunit were unchanged. Functional Na(+)-K(+)-ATPase units, measured by [(3)H]ouabain binding, undergo muscle-type specific changes. In red Gast, high-affinity ouabain-binding sites, which are a measure of alpha(2)-isozyme, increased in 30-mo-old rats despite decreased levels of alpha(2)-subunit. In white Gast, by contrast, decreased levels of alpha(2)-subunit were accompanied by decreased high-affinity ouabain-binding sites. Finally, patterns of expression of the four myosin heavy chain (MHC) isoforms (type I, IIA, IIX, and IIB) in these muscles were similar in the three age groups examined. We conclude that, in the skeletal muscles of F-344/BN rats, advancing age is associated with muscle type-specific alterations in Na(+)-K(+)-ATPase activity and patterns of expression of alpha- and beta-subunit isoforms. These changes apparently occurred without obvious shift in muscle fiber types, since expression of MHC isoforms remained unchanged. Some of the alterations occurred between middle-age (18 mo) and senescence (30 mo), and, therefore, may be attributed to aging of skeletal muscle.

In situ SR function in postinfarction myocytes.

Previous studies have shown lower systolic intracellular Ca(2+) concentrations ([Ca(2+)](i)) and reduced sarcoplasmic reticulum (SR)-releasable Ca(2+) contents in myocytes isolated from rat hearts 3 wk after moderate myocardial infarction (MI). Ca(2+) entry via L-type Ca(2+) channels was normal, but that via reverse Na(+)/Ca(2+) exchange was depressed in 3-wk MI myocytes. To elucidate mechanisms of reduced SR Ca(2+) contents in MI myocytes, we measured SR Ca(2+) uptake and SR Ca(2+) leak in situ, i.e., in intact cardiac myocytes. For sham and MI myocytes, we first demonstrated that caffeine application to release SR Ca(2+) and inhibit SR Ca(2+) uptake resulted in a 10-fold prolongation of half-time (t(1/2)) of [Ca(2+)](i) transient decline compared with that measured during a normal twitch. These observations indicate that early decline of the [Ca(2+)](i) transient during a twitch in rat myocytes was primarily mediated by SR Ca(2+)-ATPase and that the t(1/2) of [Ca(2+)](i) decline is a measure of SR Ca(2+) uptake in situ. At 5.0 mM extracellular Ca(2+), systolic [Ca(2+)](i) was significantly (P

Sprint training normalizes Ca(2+) transients and SR function in postinfarction rat myocytes.

Previous studies have shown that myocytes isolated from sedentary (Sed) rat hearts 3 wk after myocardial infarction (MI) undergo hypertrophy, exhibit altered intracellular Ca(2+) concentration ([Ca(2+)](i)) dynamics and abnormal contraction, and impaired sarcoplasmic reticulum (SR) function manifested as prolonged half-time of [Ca(2+)](i) decline. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca(2+) regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would restore [Ca(2+)](i) dynamics and SR function in MI myocytes toward normal. In MI rats, HIST ameliorated myocyte hypertrophy as indicated by significant (P

Sprint training shortens prolonged action potential duration in postinfarction rat myocyte: mechanisms.

Two electrophysiological manifestations of myocardial infarction (MI)-induced myocyte hypertrophy are prolongation of action potential duration (APD) and reduction of transient outward current (I(to)) density. Because high-intensity sprint training (HIST) ameliorated myocyte hypertrophy and improved myocyte Ca(2+) homeostasis and contractility after MI, the present study evaluated whether 6-8 wk of HIST would shorten the prolonged APD and improve the depressed I(to) in post-MI myocytes. There were no differences in resting membrane potential and action potential amplitude (APA) measured in myocytes isolated from sham-sedentary (Sed), MI-Sed, and MI-HIST groups. Times required for repolarization to 50 and 90% APA were significantly (P < 0.001) prolonged in MI-Sed myocytes. HIST reduced times required for repolarization to 50 and 90% APA to values observed in Sham-Sed myocytes. The fast and slow components of I(to) were significantly (P < 0.0001) reduced in MI-Sed myocytes. HIST significantly (P < 0.001) enhanced the fast and slow components of I(to) in MI myocytes, although not to levels observed in Sham-Sed myocytes. There were no significant differences in steady-state I(to) inactivation and activation parameters among Sham-Sed, MI-Sed, and MI-HIST myocytes. Likewise, recovery from time-dependent inactivation was also similar among the three groups. We suggest that normalization of APD after MI by HIST may be mediated by restoration of I(to) toward normal levels.

Sprint training attenuates myocyte hypertrophy and improves Ca2+ homeostasis in postinfarction myocytes.

Myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) had decreased Na+/Ca2+ exchange currents (I Na/Ca; 3 Na+ out:1 Ca2+ in) and sarcoplasmic reticulum (SR)-releasable Ca2+ contents. These defects in Ca2+ regulation may contribute to abnormal contractility in MI myocytes. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca2+ regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would ameliorate some of the cellular maladaptations observed in post-MI rats with limited exercise activity (Sed). In MI rats, HIST did not affect citrate synthase activities of plantaris muscles but significantly increased the percentage of cardiac alpha-myosin heavy chain (MHC) isoforms (57.2 +/- 1.9 vs. 49.3 +/- 3.5 in MI-HIST vs. MI-Sed, respectively; P < or = 0.05). At the single myocyte level, HIST attenuated cellular hypertrophy observed post-MI, as evidenced by reductions in cell lengths (112 +/- 4 vs. 130 +/- 5 micrograms in MI-HIST vs. MI-Sed, respectively; P < or = 0.005) and cell capacitances (212 +/- 8 vs. 242 +/- 9 pF in MI-HIST vs. MI-Sed, respectively; P < or = 0.015). Reverse I Na/Ca was significantly lower (P < or = 0.0001) in myocytes from MI-Sed rats compared with those from rats that were sham operated and sedentary. HIST significantly increased reverse I Na/Ca (P < or = 0.05) without affecting the amount of Na+/Ca2+ exchangers (detected by immunoblotting) in MI myocytes. SR-releasable Ca2+ content, as estimated by integrating forward I Na/Ca during caffeine-induced SR Ca2+ release, was also significantly increased (P < or = 0.02) by HIST in MI myocytes. We conclude that the enhanced cardiac output and stroke volume in post-MI rats subjected to HIST are mediated, at least in part, by reversal of cellular maladaptations post-MI.

Cardiac Na+,K+-ATPase isoenzymes: sensitivity to prednisolone bisguanylhydrazone.

Prednisolone-3,20-bisguanylhydrazone (PBGH), a steroid derivative, has been shown to inhibit Na+,K+-ATPase isolated from guinea-pig heart or kidney in concentrations significantly lower than those required to inhibit the enzyme obtained from other sources. Because Na+,K+-ATPases obtained from guinea-pig heart or kidney are predominantly of the alpha isoform, the hypothesis that PBGH selectively inhibits the alpha isoform over alpha (+) isoform of the enzyme was tested. Sodium dodecylsulfate polyacrylamide gel electrophoresis of the enzyme preparations revealed the presence of only the higher mobility, alpha isoform in guinea-pig heart and ferret kidney, whereas those from guinea-pig brain, dog brain and ferret heart showed both high and low mobility isoforms corresponding to alpha and alpha (+) isoforms. Na+,K+-ATPase obtained from the guinea-pig heart was most sensitive to PBGH and those isolated from ferret heart or ferret kidney had the lowest sensitivity. Enzyme preparations obtained from dog brain, dog heart or guinea-pig brain had intermediate sensitivity. This spectrum of enzyme sensitivity to PBGH was markedly different from that to ouabain. In ferret heart Na+,K+-ATPase, a low concentration of PBGH preferentially inhibited [3H]ouabain binding to the high affinity ouabain binding sites (alpha(+) isoform). These results indicate that PBGH is not a specific inhibitor of the alpha isoforms of Na+,K+-ATPase. Affinity of the enzyme for PBGH is determined by the species and tissue rather than isoforms of Na+,K+-ATPase.

Direct actions of gossypol on cardiac muscle.

Gossypol, an orally active male contraceptive, has been reported to produce cardiotoxicity. In the present study, direct actions of gossypol on the heart muscle were examined using atrial muscle preparations isolated from guinea-pig heart. Gossypol (20-30 microM) produced a transient positive chronotropic effect in spontaneously beating right atrial muscle preparations and a transient positive inotropic effect which reached a peak at about 5 min followed by a marked negative inotropic effect in paced left atrial muscle. These effects of gossypol were not affected by blocking concentrations of phentolamine, nadolol or propranolol, cimetidine and atropine. Gossypol inhibited Na+,K+-ATPase isolated from guinea-pig brain, Na+ pump activity in electrically stimulated left atrial muscle preparations of guinea-pig heart and Na+/Ca2+ exchange in Na+-loaded sarcolemmal vesicles obtained from dog heart. Inhibition of Na+/Ca2+ exchange required high concentrations of gossypol and was rapidly reversible whereas Na+,K+-ATPase inhibition was apparently irreversible. These results indicate that micromolar concentrations of gossypol have direct actions on cardiac muscle.

Depressant effects of chloroquine on the isolated guinea-pig heart.

Some anti-malarials have deleterious effects upon the heart. The actions of two of these, chloroquine and quinacrine, were compared on isolated guinea-pig atria and Langendorff preparations to assess their effects on several calcium pools. Both compounds decreased developed tension in a concentration (chloroquine 10-100 microM; quinacrine 3-100 microM) and time-dependent manner, with quinacrine being twice as potent as potent as chloroquine. Ventricular muscle was much more sensitive to chloroquine than was atrial muscle. Concentrations of chloroquine, comparable to that found in the serum of patients ingesting toxic doses, caused significant inhibition of developed tension. The effects of chloroquine were almost completely reversed on washout; quinacrine, however, was less readily reversible. Chloroquine also had a direct negative chronotropic effect, substantially reduced force-frequency relationships and developed tension in partially depolarized atrial preparations; while post-rest contraction and the positive inotropic effect of ouabain were unaffected. Increases in extracellular calcium antagonized the negative inotropic effect. Quinacrine had a marked effect on post-rest contraction and attenuated the positive inotropic action of ouabain. It is concluded that the action of chloroquine may involve a superficial calcium pool, while quinacrine may act upon several calcium pools.

Cardiodepressant actions of the orally active male contraceptive, gossypol, in guinea-pig heart muscle.

We have previously shown that gossypol has direct actions on isolated atrial muscle preparations. The possible mechanisms responsible for the transient positive and sustained negative inotropic effects of gossypol were examined under conditions that modify Ca2+ pools involved in contractile activation. In Langendorff preparations obtained from guinea-pig or rat heart, gossypol produced marked negative inotropic and arrhythmogenic effects but failed to produce a positive inotropic effect. Langendorff preparations were significantly more sensitive than atrial muscle preparations. In atrial muscle preparations, the negative inotropic effect of gossypol was not specific to utilization of superficial or intracellular Ca2+ pools; force-staircase phenomenon observed between 0.5 and 3 Hz, contractions elicited by slow action potentials in partially depolarized muscle, the inotropic effect of extracellular Ca2+ and potentiated post-rest contractions were all suppressed by gossypol to the similar extent. Low external Na+ concentrations abolished the positive inotropic effect of gossypol without affecting the negative inotropic effect. A low extracellular Ca2+ concentration enhanced the transient positive inotropic effect and delayed development of the negative inotropic effect. Simultaneous reduction of extracellular Na+ and Ca2+ concentrations abolished the positive inotropic effect and enhanced the negative inotropic effect. Gossypol inhibited ATP-dependent Ca2+ uptake by sarcolemmal vesicles obtained from dog heart. These results indicate that the actions of gossypol on cardiac muscle is not specific to utilization of either the superficial or intracellular Ca2+ pools involved in contractile activation.

High affinity and low affinity ouabain binding sites in the rat heart.

Ventricular muscle of rat heart has two classes of receptors which are responsible for the positive inotropic effect of ouabain. Low affinity receptors are apparently related to Na+, K+-ATPase. To determine if high affinity receptors are also sarcolemmal Na+, K+-ATPase of muscle cells, their characteristics were examined. Binding of [3H]ouabain to the high affinity binding site required ATP in the presence of Mg2+ and Na+, was stimulated by Na+ in the presence of Mg2+ and ATP, and was inhibited by K+. Digoxin, digitoxin and cassaine all inhibited [3H]ouabain binding to the high affinity site. Cassaine was about an order of magnitude less potent than the glycosides. These results indicate similarities in high affinity ouabain binding sites in ventricular muscle of rat heart and Na+, K+-ATPase obtained from other sources. Destruction of sympathetic nerve terminals with 6-hydroxydopamine failed to affect the high affinity ouabain binding sites indicating that high affinity sites do not represent the Na+, K+-ATPase in sympathetic nerve terminals. Labeling of Na+, K+-ATPase from [gamma-32P]ATP indicates that high affinity ouabain binding sites account for 25% of the total enzyme molecules present in ventricular muscle of rat heart.

Effects of K+ on the interaction between cardiac glycosides and Na,K-ATPase.

Inhibition of Na,K-ATPase by cardiac glycosides is at least partially antagonized by K+. The kinetics of the antagonism, however, appear complicated because K+ is capable of reducing both association and dissociation rate constants for the glycoside-enzyme interaction. In order to better understand the effect of K+, inhibition of partially purified Na,K-ATPase obtained from rat brain, guinea-pig heart and rat heart by ouabain, digoxin, digoxigenin, dihydrodigoxin and cassaine were compared in the presence of 1, 3 or 10 mM K+. Higher concentrations of K+ caused a parallel shift to the right in the concentration-inhibition curves for these compounds. For ouabain or digoxin, the extent of the shift was minimal with rat brain enzyme, intermediate with guinea-pig heart enzyme and more substantial with rat heart enzyme. For digoxigenin, dihydrodigoxin or cassaine, the extent of the shift was substantial in all enzyme preparations. These results could not be explained from either the affinity of the enzyme for the compound or its lipid solubility alone. The concentrations of these compounds required to cause a 50 percent inhibition of enzyme activity were markedly different with rat brain enzyme, but relatively similar with rat heart enzyme. The effects of K+, which depend on the source of the enzyme and chemical structures of the compounds, have to be considered in studies on comparative effects of various compounds on Na,K-ATPase, [3H]ouabain binding, sodium pumping and the force of myocardial contraction.