Intense scuba diving does not alter activity patterns of predatory reef fish: Evidence from a protected tourism hotspot.

The rise of nature-based tourism has provided a new avenue for disturbing animal behaviour, especially in protected areas. One of the most important tourism sectors in aquatic environments is scuba diving, an activity considered sustainable given its non-extractive nature and capability of bringing relevant socio-economic benefits to local communities. However, knowledge about its impact on the activity patterns of aquatic animals is still scarce. Here, we used biotelemetry techniques to assess the importance of scuba diving in modulating the activity patterns of the dusky grouper (Epinephelus marginatus, Lowe, 1834), a marine predatory fish of high interest for fishing and tourism. We implemented Hidden Markov Models (HMMs) on high-resolution acceleration data using a temporal and spatial control while controlling for a set of environmental variables (i.e. photoperiod, time-of-day, moon phase, temperature, wave height, and intensity and direction of marine currents) within a multiple-use marine protected area, and diving tourism hot-spot, of the western Mediterranean Sea. Our results underlined the more decisive influence of environmental-related stressors on the activity patterns of the dusky grouper compared to the impact of scuba diving. A high heterogeneity existed in the response against most of the stressors, including the presence of scuba divers. Overall, the activity of dusky grouper was higher at night than at day, showing a positive relationship with wave height, water temperature, and current intensity and a negative one with the moon phase. Remarkably, our findings, based on novel biotelemetry tools, differed substantially from the common wisdom accepted for this species. In conclusion, there is no clear evidence of scuba divers influence on the general activity patterns of the dusky grouper. Beyond their relevance from an ecological perspective, these results provide useful insights for the sustainable management of coastal resources, suggesting that scuba diving, when properly carried out, can represent an important sector to foster for the blue growth of coastal communities.

[Consumption of a DHA-enriched milk drink by pregnant and lactating women, on the fatty acid composition of red blood cells, breast milk, and in the newborn]. / Efecto de una bebida láctea con DHA en la composición de ácidos grasos del glóbulo rojo, de la leche materna y en el recién nacido.

UNLABELLED: EPA and DHA consumption in Chile is markedly below international daily intake recommendations. For improvement of such low intake level, we developed a fortified powdered dairy drink, containing 60 mg/serving of DHA and 14 mg/serving of EPA. OBJECTIVES: to evaluate the effects of DHA consumption during pregnancy, parturition, and in the newborn, on lipid composition of red blood cell membranes and levels of DHA and EPA on human milk during lactation. One hundred and seventy five (175) pregnant women received 2 kg/month of the fortified powdered dairy drink (GE), and 177 received the regular milk provided by the Ministry of Health (GC). We monitored daily intake of the dairy drink and all other sources of DHA and EPA in the diet, and measured general biological indicators during pregnancy, parturition, and in the new born. On a subsample of 14 women on each group (GE and GC), we measured fatty acid composition of red blood cell membranes after 0 and 60 days of consumption with the fortified dairy drink, and levels of DHA and EPA on human milk after 2 months of lactation. Of all women in the study, 16% had no consumption of fish, and 53% had no consumption of sea food. Less than 25% of all women had fish on a weekly basis. Consumption of the fortified dairy drink tripled intake of DHA, from 48.8 mg/day (CI 38.7-57.5) to 147.8 mg/day (CI 128.4-167.9) (p<0.001). The GE group had a higher content of DHA on the phospholipids of red blood cell membranes: 11.1% (CI 10.2-13.0) versus 17.9% (CI 12.9-23.0) (p<0.002). Concentration of DHA and omega-3 fatty acids increased 50% in milk of women consuming the fortified dairy drink (p<0.06). There was alower incidence of preterm parturitions on the GE group (2.8 vs 10.6%), but without statistical significance. The consumption of the fortified dairy drink produced a significant increase of DHA consumption during pregnancy and lactation, and an improved composition of red blood cell membranes, as well as in human milk, proving to be an appropriate intervention strategy for the target population.

Efecto de una bebida láctea con DHA en la composición de ácidos grasos del glóbulo rojo, de la leche materna y en el recién nacido / Consumption of a DHA-enriched milk drink by pregnant and lactating women, on the fatty acid composition of red blood cells, breast milk, and in the newborn

En Chile el consumo de EPA y DHA es mucho menor a las recomendaciones internacionales. Para mejorar el aporte de omega-3 en el embarazo y lactancia desarrollamos una bebida láctea fortificada con 60 mg de DHA y 14 mg de EPA por 200/ml. El objetivo del estudio fue evaluar su efecto en embarazo, parto y recién nacido, composición lipídica de los glóbulos rojos y de la leche materna. Se estudió una cohorte de embarazadas que recibieron dos Kg/mes del nuevo producto (GE n = 175) o la misma cantidad de la leche en polvo que distribuye el Ministerio de Salud (GC n =177). Se analizó consumo de la bebida láctea y otros alimentos fuentes de EPA y DHA y las características del embarazo, parto y recién nacido. En una submuestra de cada grupo se analizó al ingreso y a los 60 días la composición de los ácidos grasos de membranas del glóbulo rojo de la madre y de la leche materna a los 2 meses post parto. El 16 y 53 por ciento de la muestra no consumía pescado o mariscos respectivamente y menos del 25 por ciento consumía pescado en forma semanal. La intervención aumentó el aporte diario de DHA de 48,8 mg (IC 38,7 - 57,5) a 147,8 mg (IC 128,4 - 167,9) (p < 0,001). El GE tuvo mayores niveles de DHA en los fosfolípidos de la membrana del glóbulo rojo materno: 17,9 por ciento (IC 12,9 - 23,0) versus 11,1 por ciento (IC10,2 - 13,0) p < 0,002 y aumentó en 50 por ciento la concentración de ácidos grasos omega-3 en la leche materna (p 0,06). Se observó menor prevalencia de partos pretérminos en el GE (2,8 vs 10,6 por ciento), aunque sin significación estadística. La bebida láctea logró un aumento significativo del consumo de DHA en el embarazo y lactancia y una mejoría de la composición de ácidos grasos en el glóbulo rojo y leche materna, sugiriendo que es una buena estrategia de intervención en esta población.

EPA and DHA consumption in Chile is markedly below international daily intake recommendations. For improvement of such low intake level, we developed a fortified powdered dairy drink, containing 60 mg/serving of DHA and 14 mg/serving of EPA. Objectives: to evaluate the effects of DHA consumption during pregnancy, parturition, and in the newborn, on lipid composition of red blood cell membranes and levels of DHA and EPA on human milk during lactation. One hundred and seventy five (175) pregnant women received 2 kg/month of the fortified powdered dairy drink (GE), and 177 received the regular milk provided by the Ministry of Health (GC). We monitored daily intake of the dairy drink and all other sources of DHA and EPA in the diet, and measured general biological indicators during pregnancy, parturition, and in the new born. On a subsample of 14 women on each group (GE and GC), we measured fatty acid composition of red blood cell membranes after 0 and 60 days of consumption with the fortified dairy drink, and levels of DHA and EPA on human milk after 2 months of lactation. Of all women in the study, 16 percent had no consumption of fish, and 53 percent had no consumption of sea food. Less than 25 percent of all women had fish on a weekly basis. Consumption of the fortified dairy drink tripled intake of DHA, from 48.8 mg/day (CI 38.7-57.5) to 147.8 mg/day (CI 128.4-167.9) (p<0.001). The GE group had a higher content of DHA on the phospholipids of red blood cell membranes: 11.1 percent (CI 10.2- 13.0) versus 17.9 percent (CI 12.9-23.0) (p<0.002). Concentration of DHA and w-3 fatty acids increased 50 percent in milk of women consuming the fortified dairy drink (p<0.06). There was a lower incidence of preterm parturitions on the GE group (2.8 vs 10.6 percent), but without statistical significance. The consumption of the fortified dairy drink produced a significant increase of DHA consumption during pregnancy and lactation, and an improved composition...

Desarrollo, consumo y aceptabilidad de una bebida láctea con DHA para embarazadas y nodrizas / Development, intake and acceptability of a DHA-enriched milk drink for pregnant and lactating women

El DHA es un nutriente crítico en mujeres embarazadas y en lactancia, con importantes beneficios en el desarrollo cerebral y agudeza visual del lactante. En Chile el consumo de DHA representa menos del 30% de las recomendaciones internacionales para el embarazo. El objetivo del estudio fue diseñar y evaluar una bebida láctea para embarazadas y nodrizas adicionada de 60 mg de DHA y 11 de EPA por porción de consumo (200 mi), además de estar fortificada con vitaminas y minerales. Se estudió una cohorte de 177 embarazadas que recibieron 2 Kg. mensuales de bebida láctea en comparación con un grupo control de 175 embarazadas que recibió el producto actualmente en uso en el Programa Nacional de Alimentación Complementaria (leche 26% de materia grasa, fortificada con Fe, Zn y vit C). Se evaluó aceptabilidad, tolerancia y consumo de ambos productos en dos oportunidades en el embarazo y a los 2 meses posparto. Ambos alimentos fueron bien evaluados en sus características organolépticas. El consumo promedio de ambos grupos fue 1,8 +/- 1,0 tazas diarias, inferior a las 3 tazas programadas. Durante el embarazo la evaluación sensorial y el consumo fueron mayores para la leche entera, aunque las diferencias no fueron importantes desde el punto de vista biológico. Durante la lactancia no hubo diferencias en ninguno de los parámetros evaluados. Se concluye que la bebida láctea es una buena alternativa para mejorar el consumo de DHA durante el embarazo y lactancia.

DHA is a critical nutrient for pregnant and lactating women, with important functions in brain development and sight acuity of the newborn. In Chile, DHA intake represents less than 30% of the international recommendation for pregnancy. The objective of this study was to design and evaluate a milk drink for pregnant and lactating women, containing 60 mg of DHA and 11 mg of EPA per 200 mi portion, plus other added vitamins and minerals. A cohort of 177 pregnant women received 2 kg/month of the milk drink and compared to a group ofl75 women receiving the regular milk distributed by the National Complementary Feeding Program (PNAC), consisting of whole milk powder (26% fat) fortified with Fe, Zn, and Vitamin C. Evaluation included acceptability tests, tolerance, and intake of both milks, and measurements were performed at two moments during pregnancy, and after two months of lactation. Both milks had good evaluations in their organoleptic characteristies. Average intake for both groups was 1.8 +/- 1 cup per day, lower than the three cups per day that were originally defined for the study. During pregnancy, sensory evaluation was better, and intake was higher, than with the regular whole milk, although these differences were not significant from a biological standpoint. During lactation, there were no differences among any of the evaluated parameters. The developed milk drink constitutes a good alternative for improving DHA intake during pregnancy and lactation.

Reinserción laboral tras infarto agudo de miocardio (IAM): tratamiento rehabilitador frente a tratamiento convencional tras IAM / Return to work after acute myocardial infarction (AMI): rehabilitation treatment versus conventional treatment after AMI

Introducción. El infarto agudo de miocardio (IAM) es la entidad que produce mayor morbimortalidad en los países desarrollados. Afecta la calidad de vida del paciente y su situación laboral, generando un importante gasto económico. La rehabilitación cardiaca tiene entre sus metas prolongar la vida, reducir los síntomas, mejorar la función física, promover el buen estado general y facilitar la reanudación de la actividad diaria. El objetivo de este estudio es valorar la eficacia de la rehabilitación en pacientes que han sufrido un IAM en relación con la calidad de vida, la reincorporación laboral y el tiempo de baja tras el episodio coronario agudo. Material y métodos. Se seleccionaron 26 pacientes que habían sufrido un IAM. Todos siguieron el mismo tratamiento convencional. A 12 de estos pacientes se les añadió un programa de rehabilitación cardiaca. Entre 1-1,5 años después se evaluó la salud relacionada con la calidad de vida (cuestionario SF-36) y se obtuvieron parámetros relacionados con su actividad laboral. Resultados. La reinserción laboral tras 1-1,5 años del IAM mostró diferencias estadísticamente significativas a favor de los pacientes que realizaron rehabilitación. Aquellos que no han seguido el programa de rehabilitación han estado más tiempo en situación de incapacidad laboral temporal tras IAM (no estadísticamente significativa). Con excepción de la escala de dolor corporal, los pacientes que han seguido tratamiento rehabilitador tienen puntuaciones mayores en todas las demás del cuestionario SF-36; son estadísticamente significativas en 4 subescalas. Discusión. Un programa de rehabilitación tras un IAM mejora la percepción del control emocional, de la capacidad física y de la capacidad para volver al trabajo

Introduction. Acute myocardial infarction (AMI) produces more morbidity-mortality than any other disease in developed countries. It affects the patient¿s quality of life and work situation, generating significant economic cost. Among the goals of cardiac rehabilitation are: to prolong the patient¿s life, reduce symptoms, improve physical function, promote good general state and facilitate the resumption of daily activity. The aim of this study is to assess the effectiveness of rehabilitation in patients who have suffered AMI in relationship with quality of life, return to work and time of sick leave after an acute coronary episode. Material and methods. Twenty-six patients with AMI were selected. All of them received the same conventional treatment. Twelve of them were included in a cardiac rehabilitation program. Health-related quality of life (SF-36 questionnaire) was evaluated between 1-1.5 years later and the parameters related with their work activity were obtained. Results. There were statistically significant differences regarding return to work after 1-1.5 years of the AMI in favor of patients with rehabilitation (p < 0.05). Those who did not follow the rehabilitation program spent more time on sick leave for incapacity after the AMI (p > 0.05). Except for the body pain scale, those patients who had followed rehabilitation treatment had higher scores on all the other items of the SF-36 questionnaire. These were statistically significant in 4 subscales. Discussion. Rehabilitation programs after an AMI improves the patient¿s perception of his or her emotional control, physical capacity and return to work

Early infections in adult patients undergoing unrelated donor cord blood transplantation.

Early transplant-related mortality after cord blood transplantation from unrelated donors (UD-CBT) is close to 50%, mainly due to infectious complications. We have studied the incidence and characteristics of early infections (before day 100) in a series of 27 adult patients (median age 30 years, range 16-46) undergoing UD-CBT at a single institution. All 27 patients experienced at least one infectious episode and 18 (66%) suffered a severe infection. Bacteremia occurred in 55% of patients (13 with Gram-positive and 11 with Gram-negative microorganisms). Eleven of 19 CMV-seropositive patients (58%) developed CMV antigenemia and one patient had CMV disease. Fungal infections were documented in three patients (11%), comprising invasive fungal infections in two cases and a localized esophagitis in one. Ten patients (37%) died before day 100 after transplantation. Infection was considered the primary cause of death in four patients (sepsis by Acinetobacter spp. bacteremia in three cases) and contributed to death in another four. The most striking findings in this series were the high incidence of, and mortality due to multiresistant Acinetobacter spp. and the low incidence of and lack of mortality due to CMV disease. This report confirms that infection is a major complication in adults undergoing UD-CBT.

Standardized, unrelated donor cord blood transplantation in adults with hematologic malignancies.

The potential role of unrelated donor cord blood transplantation (UD-CBT) in adults remains unclear. This study reports the results of UD-CBT in 22 adults with hematologic malignancies following conditioning with thiotepa, busulfan, cyclophosphamide, and antithymocyte globulin in 21, with thiotepa, fludarabine, and antithymocyte globulin in 1, and graft-versus-host disease (GVHD) prophylaxis with cyclosporine and prednisone. Median age was 29 years (range, 18-46 years), and median weight was 69.5 kg (range, 41-85 kg). HLA match was 6 of 6 in 1 case, 5 of 6 in 13 cases, and 4 of 6 in 8 cases. Median number of nucleated cells infused was 1.71 x 10(7)/kg (range, 1.01 x 10(7)/kg to 4.96 x 10(7)/kg). All 20 patients surviving more than 30 days had myeloid engraftment, and only 1, who received the lowest cell dose, developed secondary graft failure. Median time to reach an absolute neutrophil count of at least 0.5 x 10(9)/L was 22 days (range, 13-52 days). Median time to platelets numbered at least 20 x 10(9)/L was 69 days (range, 49-153 days). Seven patients (32%) developed acute GVHD above grade II, and 9 of 10 patients at risk developed chronic GVHD, which became extensive in 4 patients. Twelve patients remained alive and disease-free 3 to 45 months after transplantation. Disease-free survival (DFS) at 1 year was 53%. Age strongly influenced DFS (P =.01). For patients aged 30 years or younger, the DFS at 1 year was 73%. These preliminary results suggest that UD-CBT should be considered a reasonable alternative in young adults with hematologic malignancy and no appropriate bone marrow donor.

Marked reduction in the incidence of hepatic veno-occlusive disease after allogeneic hematopoietic stem cell transplantation with CD34(+) positive selection.

Veno-occlusive disease of the liver (VOD) is a common and severe complication of allogeneic hematopoietic stem cell transplantation (HSCT). To determine the incidence of, and the risk factors for the development of VOD, we performed a retrospective analysis of a series of 178 patients, who underwent allogeneic HSCT at our institution between 1990 and 1999. Busulfan and cyclophosphamide constituted the conditioning regimen most frequently administered. Bone marrow was the source of stem cells in 129 patients (73%), and peripheral blood (PBSC) in 49 patients (27%). Thirty-one patients of the PBSC group received CD34(+) positively selected grafts. Most patients were given cyclosporin A and methotrexate (MTX) as graft-versus-host disease (GVHD) prophylaxis. Overall, 30 patients (17%) developed VOD. In univariate analyses, the incidence of VOD was significantly higher in recipients of unmanipulated grafts (20% vs 0%; P = 0.01), in patients with active malignant disease at transplantation (24% vs 9%; P = 0.03), in recipients of marrow from unrelated donors (33% vs 15%; P = 0.03), in patients grafted with bone marrow (21% vs 6%; P = 0.03), and in those receiving MTX as GVHD prophylaxis (21% vs 6%; P = 0.05). Under multivariate analysis, only CD34(+) positive selection (P = 0.0004) and the status of the disease at transplant (P = 0.03) were statistically significant variables for the development of VOD. We conclude that CD34(+) positively selected PBSC transplantation could result in a marked reduction in the incidence of VOD after allogeneic HSCT.

Unrelated donor cord blood transplantation in adults with chronic myelogenous leukemia: results in nine patients from a single institution.

The potential role of unrelated donor cord blood transplantation (UD-CBT) in adults is not well established. We report the results of UD-CBT in nine adult patients with chronic myeloid leukemia (CML). The median age was 27 years (range, 19-41 years), and the median weight was 62 kg (range, 45-78 kg). At transplant, six patients were in chronic phase (five in first, and one in second), two in blast crisis, and one in accelerated phase. Eight had received intensive chemotherapy, and three had undergone autologous peripheral blood hematopoietic stem cell transplantation. Four had received interferon with no cytogenetic response, and only three underwent UD-CBT within 1 year of diagnosis. After serological typing for class I antigens, and high-resolution DNA typing for DRB1, the degree of HLA match between patients and cord blood (CB) units was 4/6 in six cases and 5/6 in three cases. The median number of nucleated cells infused was 1.7 x 10(7)/kg (range, 1.2 to 4.9 x 10(7)/kg), and was above 2 x 10(7)/kg in only two cases. All patients received thiotepa, busulfan, cyclophosphamide and anti-thymocyte globulin as conditioning; cyclosporine and prednisone for graft-versus-host disease (GVHD) prophylaxis; and G-CSF from day +7 until engraftment. All seven evaluable cases engrafted. The median time to reach an absolute neutrophil count > or =0.5 x 10(9)/l and > or =1 x 10(9)/l was 22 days (range, 19-52 days) and 28 days (range, 23-64 days), respectively. In the four patients evaluable for platelet recovery time to levels of > or =20 x 10(9) platelets/l, > or =50 x 10(9) platelets/l, and > or =100 x 10(9) platelets/l, these ranged from 50 to 128 days, 60 to 139 days, and 105 to 167 days, respectively. Three patients developed acute GVHD above grade II, and three of the five patients at risk developed extensive chronic GVHD. Four patients, all transplanted in chronic phase, remain alive in molecular remission more than 18, 19, 24 and 42 months after transplantation. These preliminary results suggest that UD-CBT may be considered a reasonable alternative in adults with CML who lack an appropriate bone marrow donor.

Development of non-ABO RBC alloantibodies in patients undergoing allogeneic HPC transplantation. Is ABO incompatibility a predisposing factor?

BACKGROUND: Data from the appearance of RBC antibodies other than ABO in patients undergoing HPC transplantation are limited. STUDY DESIGN AND METHODS: The incidence and specificity of non-ABO RBC alloantibodies are described in a series of 217 patients undergoing allogeneic HPC transplantation because of various hematologic malignancies. RESULTS: Eight patients (3.7%) developed 10 antibodies after transplant. None of these patients had previously been immunized. Seven patients had one RBC antibody and one patient had three RBC antibodies. Antibody specificity were anti-Jk(b) (2 patients), -Kell (2), -M (2), -Le(b) (1), and -D (1). Finally, two patients had a panagglutinin. The mean time between transplant and antibody detection was 23 days (range, 16-672). The source of the HPCs, the conditioning regimen administered, and the type of GVHD prophylaxis administered did not influence the rate of antibody formation. On multivariate analysis, ABO blood group incompatibility (p = 0.005) and patient's age (p = 0.02) were the only two variables significantly associated with the development of RBC alloantibodies. CONCLUSION: Patients undergoing allogeneic HPC transplantation are at risk of developing RBC-specific antibodies despite the immunosuppressive therapy administered. Antibody formation was more frequently observed in ABO-mismatched cases, which suggests a potential role of this incompatibility in facilitating antibody production.

Absence of platelet response after eradication of Helicobacter pylori infection in patients with chronic idiopathic thrombocytopenic purpura.

Eradication of Helicobacter pylori infection has been associated with the correction of thrombocytopenia in patients with idiopathic thrombocytopenic purpura (ITP). We have analysed the response to eradication of H. pylori in a series of 56 adult patients with chronic ITP. Forty patients had H. pylori infection (71%) that was eradicated in 23 of 32 evaluable patients (72%). Platelet counts did not significantly vary according to H. pylori treatment outcome. Three of 56 patients (5%) achieved a partial response attributable to H. pylori eradication. Therefore, detection of H. pylori infection should not be routinely included in the initial work-up of ITP.

pi conjugation across the tetrathiafulvalene core: synthesis of extended tetrathiafulvalene derivatives and theoretical analysis of their unusual electrochemical properties

A series of extended tetrathiafulvalene (TTF) derivatives bearing one or two 1,4-dithiafulven-6-yl substitutents has been prepared. The new compounds present remarkable electrochemical singularities compared with other TTF derivatives, which are strongly affected by the nature of the substitution on the lateral heterocycle(s). This unusual electrochemical behaviour follows a square-scheme sequence and is attributed to structural changes upon oxidation of the pi-donating molecules. Digital simulations of the electrochemical data have been used to reach the values of the kinetic and thermodynamic constants involved in the square scheme. Theoretical calculations establish an important contribution of a highly delocalised resonant form involving a tetravalent sulphur in oxidised species, which could justify the occurrence of an electrochemical behaviour distinct from that of TTF. Finally, third-order susceptibilities chi 3 of two of these systems, for which electron-donating and electron-withdrawing substituents coexist and are conjugated through the TTF pi system, are given.

[The HLA system and neurofibromatosis, type I]. / HLA-System und Neurofibromatose, Typ I.

In 23 patients suffering from neurofibromatosis (von Recklinghausen's disease), we determined the serum HLA antigens. The phenotypical frequencies obtained were compared with those of 1314 people presumably healthy. Higher frequency along with a relative risk of more than 3 were found for the antigens HLA-A2, HLA-B5, HLA-B15, HLA-CW2, and HLA-CW3.

Spontaneous corneal perforation and extrusion of intraocular contents in premature infants.

Unilateral spontaneous corneal perforation with expulsion of intraocular contents is reported in two very small, high risk premature neonates. The one surviving infant demonstrated unilateral microphthalmos with aphakia and a small leukoma. The cause of this phenomenon is not clear, and measures which might be taken to prevent its occurrence are not obvious.

Determination of mercury in the presence of chloride by means of the ternary complex Hg(II)/xylenol orange/amberlite La-2 in non-aqueous media.

The system Hg(II)/Xylenol Orange/Amberlite LA-2 dissolved in 1:1 v v 3-methylbutan-1-ol/chloroform medium has been studied. A 3:2:2 complex is formed, which is suitable for analytical measurements at temperatures <18 degrees . This complex allows the determination of mercury in the range 0.19-5.5 ppm with a molar absorptivity of 2.12 x 10(4).mole(-1).cm(-1) at 600 nm and extraction pH of 7.4 (standard deviation 0.065 ppm). The proposed method has been applied to the determination of mercury in contaminated water with a high chloride content.

Calcium dependence of evoked transmitter release at very low quantal contents at the frog neuromuscular junction.

1. The relationship between the rate of evoked transmitter release and the extra-cellular concentration of Ca ions, [Ca2+]o, was studied at surface neuromuscular junctions of the frog cutaneous pectoris muscle. The average quantal content of the end-plate potential was reduced to low levels by reducing [Ca2+]o and adding 2 mM-Mn2+, 45 MM-Co2+ or 10 mM-Mg2+. 2. When the motor nerve was stimulated at a low frequency (0.5--2 Hz) in 2 mM-Mn2+ or 4 mM-Co2+, the average quantal content of evoked release was proportional to the fourth power of [Ca2+]o down to the lowest measurable quantal contents, around 2--4 quanta per 1000 stimuli. Combined with previous studies, this result indicates that evoked transmitter release has a steep, nonlinear dependence on [Ca2+]0 over our orders of magnitude of evoked release. 3. Calculations predict that if evoked and spontaneous release have the same fourth power dependence on intracellular [Ca2+], then the curve relating evoked release and [Ca2+]o should become much less steep as the evoked release rate approaches the spontaneous release rate. Our observation that the relationship between evoked release and [Ca2+]o remains fourth power down to very low release rates suggests that most spontaneous quantal release does not have the same dependence on intracellular [Ca2+], or does not use the same intracellular Ca2 pool, as evoked release. 4. In 2--10 mM-Mg2+, the lowest average quantal contents were markedly higher than the fourth power prediction. This discrepancy may occur either because Mg2+ somehow elevates intracellular [Ca2+], or because Mg2+ is itself a weak activator of transmitter release. 5. Even at very low rates of evoked release, increasing the stimulus frequency to 5--50 Hz caused a progressive increase in both evoked release and the rate of 'background' quantal release during the interstimulus interval. The frequency-dependent enhancement of both evoked and background release was more pronounced in solutions containing 10 mM-Mg2+ than in solutions containing 2 mM-Mn2+.