Hymenoptera Venom Allergy: Management of Children and Adults in Clinical Practice.

Hymenoptera venom allergy is an epidemiologically underestimated condition and a major cause of morbidity worldwide. Preventing future allergic reactions in patients who experience a systemic reaction is based on the correct management of the emergency followed by an accurate diagnosis, prescription of adrenaline autoinjectors, and, where indicated, specific venom immunotherapy. Some epidemiological studies highlight our poor knowledge of this disease and the frequent inadequacy of its management. Moreover, they emphasize the importance of such a life-saving treatment as specific immunotherapy. The availability of high-quality hymenoptera venom extracts for diagnostic and therapeutic use has dramatically improved the prognosis and quality of life of allergic patients. Subcutaneous venom immunotherapy is currently the most effective form of allergen-based immunotherapy, with a carry-over effect lasting up to several years after its interruption. This report on the management of hymenoptera venom-allergic children and adults was prepared by a panel of Italian experts. The main objective of this consensus document is to review the scientific evidence related to diagnosis, therapy, and management of patients allergic to hymenoptera venom. Thus, we can improve our knowledge of the disease and promote good clinical practices. The present document provides practical suggestions for correct diagnosis, prescription of emergency therapy and immunotherapy, and strategies for patient care.

Hymenoptera venom allergy: management of children and adults in clinical practice

Hymenoptera venom allergy is an epidemiologically underestimated condition and a major cause of morbidity worldwide. Preventing future allergic reactions in patients who experience a systemic reaction is based on the correct management of the emergency followed by an accurate diagnosis, prescription of adrenaline autoinjectors, and, where indicated, specific venom immunotherapy. Some epidemiological studies highlight our poor knowledge of this disease and the frequent inadequacy of its management. Moreover, they emphasize the importance of such a life-saving treatment as specific immunotherapy. The availability of high-quality hymenoptera venom extracts for diagnostic and therapeutic use has dramatically improved the prognosis and quality of life of allergic patients. Subcutaneous venom immunotherapy is currently the most effective form of allergen-based immunotherapy, with a carry-over effect lasting up to several years after its interruption. This report on the management of hymenoptera venom-allergic children and adults was prepared by a panel of Italian experts. The main objective of this consensus document is to review the scientific evidence related to diagnosis, therapy, and management of patients allergic to hymenoptera venom. Thus, we can improve our knowledge of the disease and promote good clinical practices. The present document provides practical suggestions for correct diagnosis, prescription of emergency therapy and immunotherapy, and strategies for patient care

La alergia al veneno de himenópteros es una condición subestimada epidemiológicamente que representa una causa importante de morbilidad en todo el mundo. La prevención de reacciones alérgicas futuras en pacientes que han desarrollado una reacción sistémica se basa en el manejo correcto de la emergencia, seguido de un diagnóstico correcto, la prescripción de autoinyectores de adrenalina y, en el caso de estar indicada, la prescripción de inmunoterapia específica con veneno (VIT). Varios estudios epidemiológicos destacan el escaso conocimiento de esta enfermedad y un frecuente tratamiento insuficiente. Además, enfatizan la importancia de la inmunoterapia específica, un tratamiento que puede salvar la vida del paciente. La disponibilidad de extractos de veneno de himenóptera de alta calidad para uso diagnóstico y terapéutico ha mejorado drásticamente el pronóstico y la calidad de vida de estos enfermos. La VIT subcutánea representa la forma más efectiva de inmunoterapia con alérgeno actualmente disponible, con una eficacia persistente que dura hasta varios años después de su interrupción. Este consenso sobre la evaluación clínica tanto de niños como de adultos alérgicos al veneno de himenópteros ha sido elaborado por un panel de expertos italianos. Su objetivo principal es revisar la evidencia científica disponible en el diagnóstico, la terapia y la evaluación clínica de los pacientes alérgicos al veneno de himenópteros con el propósito de mejorar el conocimiento sobre esta enfermedad y promover buenas prácticas clínicas. Se incluyen sugerencias prácticas para un diagnóstico correcto, la prescripción de terapia de emergencia e inmunoterapia, así como estrategias para el manejo de los pacientes

Membrane cycling after the excess retrieval mode of rapid endocytosis in mouse chromaffin cells.

AIM: After exocytosis, neuroendocrine cells and neurones keep constant the plasma membrane and the releasable vesicle pools by performing endocytosis and vesicular cycling. Patch-clamp capacitance measurements on chromaffin cells showed that strong Ca(+2) entry activates excess retrieval: a rapid endocytosis process that retrieves more membrane than the one fused by preceding exocytosis. The main purpose of the present experiments was to study the recycling pathway that follows excess retrieval, which is unknown. METHODS: Membrane recycling after exocytosis-endocytosis can be studied by fluorescence imaging assays with FM1-43 (Perez Bay et al. Am J Physiol Cell Physiol 2007; 293, C1509). In this work, we used this assay in combination with fluorescent dextrans and specific organelle-targeted antibodies to study the membrane recycling after excess retrieval in mouse chromaffin cells. RESULTS: Excess retrieval was observed after the application of high-K(+) or cholinergic agonists during 15 or 30 s in the presence of FM1-43. We found that the excess retrieval membrane pool (defined as endocytosis-exocytosis) was associated with the generation of a non-releasable fraction of membrane (up to 30% of plasma membrane surface) colocalizing with the lysosomal compartment. The excess retrieval membrane pool followed a saturable cytosolic Ca(2+) dependency, and it was suppressed by inhibitors of L-type Ca(2+) channels, endoplasmic reticulum Ca(2+) release and PKC. CONCLUSION: Excess retrieval is not associated with the cycling of releasable vesicles, but it is related to the formation of non-releasable endosomes. This process is activated by a concerted contribution of Ca(2+) entry through L-channels and Ca(2+) release from endoplasmic reticulum.

Comparison between continuous or intermittent schedules of sublingual immunotherapy for house dust mites: effects on compliance, patients satisfaction, quality of life and safety.

Sublingual immunotherapy (SLIT) is indicated in the treatment of allergic rhinitis and asthma. However, an issue scantly investigated is the patients satisfaction and the consequent compliance. This study is aimed at evaluating the possible differences of SLIT administered continuously or intermittently on several parameters: clinical efficacy, Quality of Life (QoL), satisfaction, compliance and safety. Forty allergic patients were treated for 12 months. The treatment was carried out by sublingual administration of an allergen extract of a 50% mixture of Dermatophagoides pteronyssinus and Dermatophagoides farinae at 10 and 300 IR/ml concentrations. Patients were randomly treated continuously or intermittently (i.e. 2 month treatment alternate to 2 month suspension). Both schedules were significantly effective in reducing allergic symptoms and improving QoL. Compliance and satisfaction were good in both groups. Local and systemic reactions were few, self-resolving, and mild in both schedules. Intergroup analysis did not reveal any difference between the two groups regarding these parameters. In conclusion, this preliminary study provides the evidence that also intermittent SLIT is as effective and safe as traditional continuous treatment. In addition, compliance and satisfaction are super-imposable in the two groups.

Characteristics of patients with allergic polysensitization: the POLISMAIL study.

BACKGROUND: The natural history of respiratory allergy is commonly characterized by a worsening of symptom severity, frequent comorbidity of rhinitis and asthma, and polysensitization to aeroallergens. The polysensitization phenomenon starts since childhood and is rare to find monosensitized adult patients. However, there are few studies investigating the characteristics of polysensitized patients. METHODS: This study was performed on a large cohort of patients with allergic rhinitis (assessed by ARIA criteria) and/or mild to moderate asthma (assessed by GINA). The kind and the number of sensitizations, their patterns, and the relation with quality of life (QoL) measured by the Juniper's RQLQ guestionnaire, were evaluated. RESULTS: Globally 418 patients (50.2% males, 49.8% females, mean age 26.4 years, range 3.5-65 years, 64 smokers, 371 non-smokers) were enrolled: 220 had allergic rhinitis alone, and 198 allergic rhinitis and asthma. The mean number ofsensitizations was 2.6. Three hundred-five patients (73%) had persistent rhinitis (PER), 220 of them with moderate-severe form. There was no significant derence in rate of rhinitis and asthma in monosensitized or polysensitized patients. Most patients were sensitized to pollens, whereas only 24.2% of them were sensitized to perennial allergens. Polysensitization was significantly associated with some issues of QoL, confirming previous findings, but not with number ofsensitizations. CONCLUSIONS: This study provides data confirming for poly-sensitized patients the relevance of ARIA classification of AR. PER is the most common form of AR in this cohort, symptoms are frequently moderate-severe, and asthma is present in about the half of patients with AR.

Increased CD34+ cell peripheral traffic appears to be an unique feature of the allergic inflammation.

Evidence has been cumulated during the last years concerning the immaturity of the cells involved in the local and systemic aspects of allergic inflammation. Hematopoietic precursors (HPC) are mobilized from the bone matrix as multipotent cells or, more often, as progenitors that, after the initial white-lineage commitment reach through the peripheral blood (PB) their final destinations constituted by the target organs of allergy. Although several studies have investigated the CD34+ cells traffic and location at the level of the inflamed peripheral mucosae in allergic populations, limited information is available on their behaviour on the time-course of infectious diseases. The current study thus was designed to asses the peripheral traffic of CD34+ HPC during the infectious inflammation. To this end CD34+ HPCs have been enumerated, by flow-cytometric techniques, in PB of 24 adult healthy beings (Group A), 24 adult subjects with symptomatic extrinsic allergy (Group B) and in PB of 24 adult patients hospitalised for febrile infectious pathology (Group C). CD34+ cell values ranged 0.01-0.08% with a median of 0.03 in Group A. In Group B values ranged 0.17-0.75% with a median of 0.28 and in Group C values ranged 0.00-0.12% with a median of 0.07. Variance analysis test among the three groups was statistically significant (p<0.001) supporting the conclusion that CD34+ HPC mobilizing and increased peripheral traffic is an unique feature of the allergic inflammation.

Higher frequency of early local side effects with aqueous versus depot immunotherapy for hymenoptera venom allergy.

Venom immunotherapy has proven a very effective method for the treatment of allergy to Hymenoptera venom. Aqueous instead of depot extracts are prevalently used for this immunotherapy. The advantage of using aqueous extracts has not been fully investigated. We made an open, non-controlled study on 45 subjects sensitized to either Apis mellifera or Vespula spp. Patients were assigned to either a depot (N=27) or an aqueous (N=18) immunotherapy regimen, and side effects were monitored during the induction and the 3-year maintenance phase. The effect of naturally occurring stings during the treatment and after its interruption was recorded as well. Side effects were less frequent with the depot extract both on a "per patient" (22.2% versus 50.0%) and on a "per dose" (2.9% versus 10,2%) basis (p=0.026 and p<0.000, respectively). Better tolerance was mainly due to the lower frequency of local side effects occurring at early times after vaccination. The efficacy of vaccination was comparable in the 2 cohorts, as expected. We conclude that depot immunotherapy to Hymenoptera venom should be preferred to aqueous immunotherapy for the lower occurrence of local side effects. This might influence a better compliance with this potentially life-saving treatment.

Lithium and KB-R7943 effects on mechanics and energetics of rat heart muscle.

The role of calcium influx on energy expenditure during cardiac contraction was studied. For this purpose, the described ability of lithium and KB-R 7943 (KBR) to diminish Ca entry through Na-Ca exchanger (Ponce-Hornos & Langer, J Mol Cell Cardiol 1980, 12, 1367, Satoh et al., Circulation 2000, 101, 1441) were used. In isolated contractions (contractions elicited after at least 5 min of rest) LiCl 45 mmol L(-1) decreased pressure developed and pressure-time integral from 42.3 +/- 2.7 and 14.5 +/- 1.2 to 32.1 +/- 3.4 mN mm(-2) and 8.3 +/- 0.9 mN mm(-2) s, respectively. A similar effect was observed in regular contractions (at 0.16 Hz stimulation). The presence of KBR (5 micromol L(-1)) in the perfusate induced a slight but not significant decrease in pressure developed and pressure-time integral in steady-state contractions. As it was previously described, the heat involved in a heart muscle contraction can be decomposed into several components (H1, H2, H3 and H4), but only one (H3) was associated with force generation. While H3 decreased with lithium in both types of contractions, H3/PtI ratio remained unaltered, indicating that the economy for pressure maintenance was unaffected. To further investigate the role of Ca entry on force development, a condition in which the contraction is mainly dependent on extracellular calcium was studied. An 'extra' stimulus applied 200 ms after the regular one in a muscle stimulated at 0.16 Hz induces a contraction with this characteristic (Marengo et al., Am J Physiol 1999, 276, H309). Lithium induced a strong decrease in pressure-time integral and H3 associated with this contraction (43 and 45%, respectively) with no change in H3/PtI ratio. Lithium also reduced (53%) an energy component (H2) associated with Ca cycling. The use of KBR showed qualitatively similar results [i.e. a 33% reduction in pressure-time integral associated with the extrasystole (ES) with no changes in H3/PtI ratio and a 30% reduction in the H2 component]. Li and KBR effects appear to be additive and in the presence of 45 mmol L(-1) Li and 5 micromol L(-1) KBR the extrasystole was abolished in 77%. Lithium and KBR effects particularly for the extrasystole can be explained through the inhibition of Ca entry via Na-Ca exchange giving support to the participation of the Na-Ca exchanger in the Ca influx from the extracellular space. In addition, the results also suggest the possibility of an effect of Li on an additional Ca sensitive locus (different than the Na-Ca exchanger). In this connection, in isolated contractions lithium decreased the energy release fraction related to mitochondrial processes (H4) increasing the economy of the overall cardiac contraction.

Development and dissipation of Ca(2+) gradients in adrenal chromaffin cells.

We used pulsed laser imaging to measure the development and dissipation of Ca(2+) gradients evoked by the activation of voltage-sensitive Ca(2+) channels in adrenal chromaffin cells. Ca(2+) gradients appeared rapidly (<5 ms) upon membrane depolarization and dissipated over several hundred milliseconds after membrane repolarization. Dissipation occurred with an initial fast phase, as the steep gradient near the membrane collapsed, and a slower phase as the remaining shallow gradient dispersed. Inhibition of active Ca(2+) uptake by the endoplasmic reticulum (thapsigargin) and mitochondria (carbonylcyanide p-trifluoro-methoxyphenylhydrazone/oligomycin) had no effect on the size of Ca(2+) changes or the rate of gradient dissipation, suggesting that passive endogenous Ca(2+) buffers are responsible for the slow Ca(2+) redistribution. We used a radial diffusion model incorporating Ca(2+) diffusion and binding to intracellular Ca(2+) buffers to simulate Ca(2+) gradients. We included a 3D optical sectioning model, simulating the effects of out-of-focus light, to allow comparison with the measured gradients. Introduction of a high-capacity immobile Ca(2+) buffer, with a buffer capacity on the order of 1000 and appropriate affinity and kinetics, approximated the size of the Ca(2+) increases and rate of dissipation of the measured gradients. Finally, simulations without exogenous buffer suggest that the Ca(2+) signal due to Ca(2+) channel activation is restricted by the endogenous buffer to a space less than 1 microm from the cell membrane.

The heart extrasystole: an energetic approach.

The consequences of an extrasystole (ES) on cardiac muscle's energetics and Ca2+ homeostasis were investigated in the beating heart. The fraction of heat release related to pressure development (pressure dependent) and pressure-independent heat release were measured during isovolumic contractions in arterially perfused rat ventricle. The heat release by a contraction showed two pressure-independent components (H1 and H2) of short evolution and a pressure-dependent component (H3). The additional heat released by ES was decomposed into one pressure-independent (H'2) and one pressure-dependent (H'3) component with time courses similar to those of control components H2 and H3. ES also induced the potentiation of pressure development (P) and heat release during the postextrasystolic (PES) beat. The slope of the linear relationship between pressure-dependent heat and pressure maintenance was similar in control, ES, and PES contractions (0.08 +/- 0.01, 0.10 +/- 0.02, and 0.08 +/- 0.01 mJ. g-1. mmHg-1. s-1, respectively). The potentiation of H2 (heat component related with Ca2+ removal processes) in PES was equal to H'2 at 0.3, 0.5, 1, and 2 mM Ca2+, suggesting that the extra amount of Ca2+ mobilized during ES was recycled in PES. Pretreatment with 1 mM caffeine to deplete sarcoplasmic reticulum Ca2+ content inhibited both the mechanical and energetic potentiation of PES. However, the heat released and the pressure developed during ES were not changed by sarcoplasmic reticulum depletion. The results suggest that 1) the source of Ca2+ for ES would be entirely extracellular, 2) the Ca2+ entered during ES is accumulated in the sarcoplasmic reticulum, and 3) the Ca2+ stored by the sarcoplasmic reticulum during ES induces an increased contribution of this organelle during PES compared with the normal contraction.

Effects of long-term smoking on myocardial blood flow, coronary vasomotion, and vasodilator capacity.

BACKGROUND: The effect of long-term smoking on coronary vasomotion and vasodilator capacity in healthy smokers is unknown. METHODS AND RESULTS: Myocardial blood flow (MBF) was quantified with [13N]ammonia and positron emission tomography (PET) at rest, during cold pressor testing (endothelium-dependent vasomotion), and during dipyridamole-induced hyperemia in 16 long-term smokers and 17 nonsmokers. MBF at rest did not differ between the 2 groups. Cold induced similar increases in rate-pressure product (RPP) in smokers and nonsmokers. However, MBF increased only in nonsmokers and was, during cold, higher than in smokers (0.91+/-0.18 versus 0.78+/-0.14 mL x g(-1) x min(-1), P<0.05). MBF normalized to the RPP (derived from the ratio of MBF ([milliliters per gram per minute] to RPP [beats per minute times millimeters of mercury] times 10000) declined in smokers but remained unchanged in nonsmokers (0.86+/-0.10 versus 0.72+/-0.11, P=0.0006, and 0.99+/-0.25 versus 0.96+/-0.27, P=NS). The hyperemic response to dipyridamole and the myocardial flow reserve did not differ between the 2 groups. In a multiple regression model adjusted for age, sex, serum lipid levels, years of smoking, and pack-years, years of smoking was the strongest predictor of the normalized blood flow response to cold (P<0.001), followed by the HDL/LDL ratio. CONCLUSIONS: The normal hyperemic response to dipyridamole in long-term smokers indicates a preserved endothelium-independent coronary vascular smooth muscle relaxation, whereas the abnormal response to cold suggests a defect in coronary vasomotion likely located at the level of the coronary endothelium. Its severity depends on the total exposure time to smoking.

Dependence of cardiac cell Ca2+ permeability on sialic acid-containing sarcolemmal gangliosides.

The specific removal of negatively-charged sialic acid by neuraminidase produces a large increase in cardiac myocyte Ca uptake (17.3 +/- 1.1 mmol Ca/kg dry weight) and marked cell contracture. Importantly, the insertion of the negatively-charged amphiphile dodecyl sulfate in the sarcolemma eliminates the increased calcium uptake and preserves contractile function. In the present study, we further examine the role of sialic acid-Ca interaction and, specifically, the role of gangliosides (sialic acid-containing glycolipids) in cardiac cells' Ca permeability. Neonatal cell culture and adult ventricular myocytes were used. The major findings of this study are: (1) while dodecyl sulfate inhibits cellular calcium uptake and contracture development induced by sialic acid removal, cationic and neutral amphiphiles are without effect. (2) Ca channel blockers (nifedipine and protamine) and the Na/Ca exchange inhibitor Ni do not modify the effect of sialic acid removal. (3) A non-classical-channel related whole-cell current appears and increases after 21 +/- 2.2 min treatment with 0.02 U/ml neuraminidase (n = 4). Incubation with neuraminidase in the presence of dodecyl sulfate greatly delays the appearance of these currents to 44.4 +/- 6.1 min (n = 4). (4) The use of a specific probe for GM1 ganglioside, the cholera toxin B subunit (3 micrograms/ml), induces a moderate but clear increase in cellular Ca (1.63 +/- 0.3 mmol Ca kg dry weight; n = 8). However, this increase was not modified by treatment with dodecyl sulfate. (5) Finally, 50 mU/ml endoglycoceramidase, an enzyme which specifically cleaves the link between the sialic acid-containing oligosaccharide and ceramide of gangliosides, induced a significant increase in Ca uptake (4.4 +/- 0.9 mmol Ca kg dry weight, n = 4). These results indicate the importance of negatively charged sialic acid-containing gangliosides in the maintenance of cardiac cell physiological Ca permeability. The increase in Ca uptake induced by the removal of sialic acid seems to be mediated by development of a Ca "leak" via other than classical cation channels.

The effects of temperature upon calcium exchange in intact cultured cardiac myocytes.

Cardiac myocyte Ca transport systems, such as sarcoplasmic reticulum Ca-ATPase and sarcolemmal Na/Ca exchange (Na/Ca), are critically dependent on temperature. The purpose of this work was to study the effect of temperature on cellular Ca compartmentation and its exchange characteristics in intact functional neonatal cultured myocytes. The Na/Ca mediated Ca exchange (CaNa/Ca)--including its sarcoplasmic reticulum (SR) and sarcolemmal (SL) contributions, a slow exchange component related to mitochondrial Ca and the La displaceable Ca pool were studied combining isotopic and gas-dissection techniques for membrane isolation. The major findings of this study are: (i) The amount of Ca exchanged through CaNa/Ca is clearly dependent on temperature (Q10 approximately 1.6) in the range studied (17-37 degrees C); (ii) the addition of 1 microM nifedipine does not modify the temperature dependence of CaNa/Ca; (iii) the sarcolemmal bound fraction contributing to CaNa/Ca is not changed by temperature; (iv) the increase in CaNa/Ca with temperature is explained by an increment in the contribution of SR-Ca to CaNa/Ca; (v) a fraction of SR which does not exchange via CaNa/Ca at low temperatures can be released and mobilized by caffeine-this caffeine sensitive fraction is reduced as temperature is increased and is no longer measurable as a separate entity at 37 degrees C; (vi) if we consider (iv) and (v) together, SR content would be temperature dependent with a Q10 of approximately 1.5; (vii) a La displaceable pool, which represents over 66% of the total exchangeable Ca, increases in the range of 22-33 degrees C with a Q10 of 1.25 which is consistent with a pool distribution of 70% SL-bound and 30% SR-derived [Post J.A., Langer G.A. Cellular origin of the rapidly exchangeable calcium pool in the cultured neonatal rat heart cell. Cell Calcium 1992; 13: 627-634]; and (viii) the rate constant for the mitochondrial Ca component increases by 60% from 22 degrees C to 37 degrees C, but Ca content in this organelle is not modified over this temperature range.

Tension-dependent and tension-independent energy components of heart contraction.

Heat production and isovolumetric pressure development (P) were measured simultaneously in the arterially perfused rat ventricle. The time course of the calorimetric signal that follows a contraction could be decomposed into four components of energy released. Three of these components (H1, H2, and H4) were pressure independent, only H3 correlated with either P or the pressure-time integral (PtI) (r > 0.78, n = 36, P < 0.01). The dimensionless slope of the regression of H3 on P was 0.24 (an index of muscle economy) and the absence of O2 (N2 replacement) decreased it to 0.178 suggesting that 26% of H3 is related to oxidative metabolism. H4 was the most affected by the lack of O2 in the perfusate. It decreased to 16% in the first beat under N2 without change in P or in H1, H2 or H3, and disappeared (1.6 +/- 1.0 mJ.g-1) in the fourth contraction under N2 (while P, H1, H2 and H3 remained over 64% of their control values). H4 was activated during the first 1-3 beats after a quiescent period and remained active for several seconds (even in the absence of subsequent stimulation) as if the basal metabolism had been increased to a new steady level. H1 and H2 were dependent on the extracellular Ca. The magnitudes of both H1 (1.8 +/- 0.2 mJ.g-1) and H2 (2.7 +/- 0.2 mJ.g-1) were similar to those reported for the fast and slow components of activation heat in skeletal muscle. If twin stimuli are applied (200 ms apart), additional energy is released (+3.0 +/- 0.3 mJ.g-1) that can be decomposed in two components similar to those identified as H2 and H3. The magnitude of H1, its absence in the twin contraction and its Ca dependency suggest an association with Ca-binding processes (mainly Troponin C). The presence of an H2 component during the twin contraction, its magnitude and Ca dependence gives support to a relationship between H2 and Ca removal processes.

Mechanisms underlying post-rest potentiation in isolated left rat atria.

In the isolated rat left atria the influence of rest intervals on the force developed by the first post rest beat (PRB) was studied. Under control conditions the force developed by the PRB increased (respect to previous steady state at 0.5 Hz) with the increase of the rest interval until 20 sec of pause and decreased with longer intervals. In the presence of caffeine (1 or 4 mM plus high [Ca]0) there was a monotonous fall of the PRB as a function of the rest interval. When extracellular calcium was replaced by Sr the tension developed by PRB vs. rest interval curve rose with a slope lower than the control one and reached the peak at 60 sec. At saturation levels of [Ca]0 the PRB tension development did not vary up to 20 sec pause but the decreasing phase observed after 20 sec of rest interval was still present. At 0.5 mM [Ca]0 the response was similar to control curve. The results in the presence of caffeine and strontium suggest that, in rat atria, the rest potentiation appears to be dependent on the release of calcium from intracellular stores (sarcoplasmic reticulum). This is consistent with the hypothesis proposing that longer resting periods provide a longer interval for the transfer of Ca from uptake to release sites in the sarcoplasmic reticulum.

Mechanisms underlying post-rest potentiation in isolated left rat atria.

In the isolated rat left atria the influence of rest intervals on the force developed by the first post rest beat (PRB) was studied. Under control conditions the force developed by the PRB increased (respect to previous steady state at 0.5 Hz) with the increase of the rest interval until 20 sec of pause and decreased with longer intervals. In the presence of caffeine (1 or 4 mM plus high [Ca]0) there was a monotonous fall of the PRB as a function of the rest interval. When extracellular calcium was replaced by Sr the tension developed by PRB vs. rest interval curve rose with a slope lower than the control one and reached the peak at 60 sec. At saturation levels of [Ca]0 the PRB tension development did not vary up to 20 sec pause but the decreasing phase observed after 20 sec of rest interval was still present. At 0.5 mM [Ca]0 the response was similar to control curve. The results in the presence of caffeine and strontium suggest that, in rat atria, the rest potentiation appears to be dependent on the release of calcium from intracellular stores (sarcoplasmic reticulum). This is consistent with the hypothesis proposing that longer resting periods provide a longer interval for the transfer of Ca from uptake to release sites in the sarcoplasmic reticulum.