The functional refractory period of atrioventricular (AV) transmission has been accepted as a measure of AV nodal refractoriness and has been assumed to be determined solely by conduction of interpolated extrasystoles through the AV node when it is partially refractory. In the present study, we found an important effect of the conduction time of the regular beats by measuring AV nodal conduction times of atrial extrasystoles from the His bundle of isolated, blood-perfused dog hearts. We separated three independent components that determine AV nodal conductivity: (1) a "basal conduction time" measured with a "postmature" extrasystole at low heart rates (<120 /min), (2) a rate-dependent increment in conduction time (previously called "fatigue") that affects both normal and premature cycles equally and (3) an exponential change in conduction time that depends entirely on the immediately preceding interval and, therefore, is not further affected by heart rate. The functional refractory period is one point defining this continuous exponential function. We showed that an important cause of the decrease in the functional refractory period that occurs when heart rate is increased is the change in the conduction time of the regular beats. KEY WORDS electrophysiologyHis bundle extrasystole conductivity dog heart rate epinephrine
The effects of stimulating vasoconstrictor fibres and infusing noradrenaline and vasopressin on intestinal vascular resistance were studied in cats, using either constant flow or constant pressure. The responses to stimulating constrictor fibres and to infusing noradrenaline were essentially the same whether pressure or blood flow was kept constant, and the earlier described autoregulatory escape was of about the same magnitude during both these conditions. The results further support the hypothesis that a redistribution of blood flow takes place within the intestinal wall, and the combined evidence suggests an increased submucosal flow at the expense of the mucosa. The similarities between infusing noradrenaline and stimulating constrictor fibres suggest that the vasoconstrictor fibre endings and the α‐receptors may have a differentiated distribution in the intestinal bed. This view is supported by the fact that another vasoconstrictor agent, vasopressin, produces a well sustained vasoconstrictor response without secondary adjustments. Some similarities between ‘autoregulatory escape’ during nerve stimulation and autoregulation of blood flow at different pressures are pointed out. It is further emphasized that both these phenomena are very often diminished, distorted or even totally suppressed by the introduction of an artificial pump system.
DRESEL, P., B. FOLKOW and I. WALLENTIN. Rubidiumn6 clearance during neurogenic redistribution of intestinal bloodJlow. Acta physiol. scand. 1966. 67. 173-184. Arterio-venous extraction of rubidiumse was recorded in the intestine of the cat, using two different administration techniques; and the capillary transport coefficient, PS, was calculated. I n all experiments intestinal blood flow resistance was calculated, and in some the capillary filtration coefficient, CFC. These two methods to determine the capillary surface area open to blood flow were well correlated, but the total range for variations for PS was less than for CFC. This difference was explained by the fact that exclusion of some part of the capillary circulation by the precapillary sphincters causes the filtration to cease much more completely than the diffusion. Therefore, CFC seems to better reflect the changing capillary surface area than PS. During the "autoregulatory escape", the typical vascular response to stimulation of the vasoconstrictor fibres, PS was decreased but in most cases much less so than CFC. These results seem to exclude involvement of true arteriovenous shunts to any significant degree during activation of the vasoconstrictor nerves. If the redistribution of blood flow from the mucosa towards the submucosa during the "autoregulatory escape" utilizes special low-resistance channels, these must allow for exchange of solutes, implying a situation of "physiological shunting" rather than "true-shunting".
The relationship between cellular calcium (Ca) stores and isometric contractile force was investigated in isolatcd, gas-pcrfuscd cat hearts.
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