Iceberg discharges into the north atlantic on millennial time scales during the last glaciation.

High-resolution studies of North Atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years, much more frequently than the 7000-to 10,000-year pacing of massive ice discharges associated with Heinrich events. The calving cycles correlate with warm-cold oscillations, called Dansgaard-Oeschger events, in Greenland ice cores. Each cycle records synchronous discharges of ice from different sources, and the cycles are decoupled from sea-surface temperatures. These findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above Greenland and in calving from more than one ice sheet.

Changes in Atmospheric Circulation and Ocean Ice Cover over the North Atlantic During the Last 41,000 Years.

High-resolution, continuous multivariate chemical records from a central Greenland ice core provide a sensitive measure of climate change and chemical composition of the atmosphere over the last 41,000 years. These chemical series reveal a record of change in the relative size and intensity of the circulation system that transported air masses to Greenland [defined here as the polar circulation index (PCI)] and in the extent of ocean ice cover. Massive iceberg discharge events previously defined from the marine record are correlated with notable expansions of ocean ice cover and increases in PCI. During stadials without discharge events, ocean ice cover appears to reach some common maximum level. The massive aerosol loadings and dramatic variations in ocean ice cover documented in ice cores should be included in climate modeling.

Effect of arteriovenous shunting on plasma vasopressin levels in the anesthetized dog.

Studies were performed in dogs anesthetized with chloralose to establish the plasma arginine vasopressin (AVP) response to 30-minute periods of femoral arteriovenous (AV) shunting at levels of either 10%, 30%, or 50% of control cardiac output. Significant increases in cardiac output, heart rate, and total body peripheral resistance (shunt resistance excluded), suggesting decreased sinoaortic baroreceptor activity, occurred at shunt fractions greater than 10%. There were no significant changes in plasma concentrations of AVP at any level of AV shunting. In separate studies, significant elevations in plasma AVP levels were observed in vagotomized dogs during AV shunting of 50% of control cardiac output. These results indicate that AV shunting less than or equal to 50% of cardiac output does not acutely alter plasma AVP levels in the intact dog and that cardiopulmonary receptors with vagal afferents suppress AVP release that might otherwise occur in this canine model of AV shunting.

Measurement of vitamin B12 in the livers and sera of sheep and cattle and an investigation of factors influencing serum vitamin B12 levels in sheep.

Modifications of a radioassay method for the analysis of vitamin B12 using chicken serum as the binder are described. This obviates the need to use individual serum blanks to correct for non-specific binding in vitamin B12 assays of the sera and livers of sheep and cattle. Samples with high vitamin B12 levels can be diluted prior to assay without loss of linearity. Recoveries of added cyanocobalamin or hydroxocobalamin were better than 95% and results correlated significantly with those obtained using a microbiological assay (Poteriochromonas malhamensis). Sera and liver samples stored for four weeks at temperatures ranging from -20 degrees to 22 degrees showed no change in vitamin B12 levels. Withholding food from sheep for 44 hours led to a marked increase in serum vitamin B12. This effect was also evident in sheep eating a limited amount of cut grass. In sheep at pasture there was no evidence of a diurnal variation in serum vitamin B12 levels. Serum vitamin B12 levels in sheep at pasture were shown to be an unreliable indicator of liver vitamin B12.

The distribution of copper, vitamin B12 and zinc in the livers of sheep.

Six livers from one-year-old sheep were each sampled at 27 different sites and the concentrations of copper, vitamin B12 and zinc determined for each sample. The distribution of these components was uniform throughout the liver, indicating that analysis of a single liver biopsy sample would give an accurate assessment of their hepatic concentration.

Role of renal nerves in mediating the increased renin secretion during continuous positive-pressure ventilation.

The effect of renal denervation on the increase in plasma renin levels during continuous positive-pressure ventilation (CPPV) was studied in anesthetized dogs. Renin secretion was measured in a group of dogs with intact renal nerves and a group which had previously undergone bilateral renal denervation. Renal perfusion pressure was maintained constant throughout the experiment. Renin secretion increased significantly during application of CPPV with a positive and expiratory pressure of 10 cm water in the dogs of the intact group but was not altered in dogs of the denervated group. These results support the conclusion that CPPV increases renin secretion by reflexly altering renal nerve activity.

Role of sinoaortic baroreceptors in initiating the renal response to continuous positive-pressure ventilation in the dog.

Experiments were conducted to determine the role of cardiopulmonary receptors with vagal afferents and sinoaortic baroreceptors in initiating the reflex antidiuresis and antinatriuresis during continuous positive-pressure ventilation (CPPV). CPPV was applied to 18 dogs: seven control, five that underwent bilateral vagotomy; and six that underwent sinoaortic baroreceptor denervation. The dogs were anesthetized with pentobarbital, paralyzed with succinylcholine, and their lungs were mechanically ventilated with a volume ventilator. Renal function and systemic hemodynamics were monitored by clearance methods and pressures, respectively. After two 30-min control periods of intermittent positive-pressure ventilation (IPPV), CPPV using 10 cm H2O positive end-expiratory pressure was applied for two 30-min experimental periods. Three 30-min recovery periods of IPPV followed. In dogs of the control group and vagotomy group, CPPV caused statistically significant decreases from control levels in urinary flow, sodium excretion, and glomerular filtration rate. However, in the dogs of the sinoaortic baroreceptor denervation group, CPPV did not produce any significant change in these variables. Therefore, the results do not support the hypothesis that cardiopulmonary receptors mediate the renal response to CPPV. Rather, the data indicate that the aortic arch and carotid sinus baroreceptors participate in initiating the reflex antidiuresis and antinatriuresis during CPPV.

Integration of chemoreceptor stimuli by rostral brainstem respiratory areas.

Ventilatory regulation by brainstem sites rostral to the midpontile level was assessed in decerebrate cats by comparing the effects of punctate pneumotaxic center lesions with those of midpontile transection. After either procedure, PACO2 was significantly elevated. Moreover an equal suppression of hypercapnia-induced minute volumes and maintenance, at some PACO2 levels, of minute volume responses to hypoxia was observed. Tidal volume elevations accounted for the maintenance of hypoxia-induced minute volumes. Following pneumotaxic center lesions, hypercapnia-induced tidal volumes were higher than those exhibited subsequent to midpontile transection. After carotid sinus nerve section, PACO2 was elevated and hypoxia-induced alterations were abolished. Bilateral vagotomy resulted in apneusis. These data demonstrate that, in the brainstem area examined, only the pneumotaxic center influences the PACO2 level or set point for respiratory activity. A locus of tidal volume generation is ascribed to rostral brainstem sites outside this pneumotaxic center. Data obtained support the hypothesis of a differential brainstem integration of peripheral and central chemoreceptor afferent stimuli.

Atrial receptors and renal function.

The hypothesis that receptors in the heart or pulmonary vasculature initiate a reflex that influences urine flow was derived from experiments designed to evaluate the effect of mechanical ventilation on renal function. These experiments indicated that urine flow usually decreases during positive-pressure breathing and usually increases during negative-pressure breathing. It was surmised that impulses from certain cardiopulmonary receptors affect the secretion of ADH, which in turn influences urine flow. A subsequent investigation appeared to localize the pertinent receptors to the left atrium, but the results of this particular investigation were influenced by several complication factors that have not been widely appreciated. The apparent localization of volume-regulating recpetors to the left atrium and the accumulating evidence that atrial receptors do respond to changes in atrial pressure or atrial volume triggered a myriad of further studies on the function of left receptors. Nearly all these studies employed indirect techniques that produced changes in systemic and pulmonary hemodynamics in addition to changes in left atrial pressure. Nevertheless, it often was assumed that if changes in left pressure were produced, any concomitant changes in circulating ADH or in urine flow were attributable to a reflex elicited from atrial receptors. Mush of the data obtained were interpreted as being compatible with the elft atrial volume-receptor hypothesis, but very liggle of the data pertained to left atrial receptors specifically.