A comparative quantitative study of the effects of dihydroergotamine (DHE)and noradrenaline (NA) on the precapillary and postcapillary resistance vessels, the precapillary 'sphincters', and the capacitance vessels was performed in a skeletal muscle and a skin region in healthy humans and in patients with orthostatic symptoms; further, the vascular effects of the drugs were analysed in muscle, skin, intestine and kidney in cats before and after sympathectomy.2. The two drugs evoked a similar pattern of response in the cutaneous vascular bed, i.e. they both constricted resistance and capacitance vessels, increased the ratio of pre-/post-capillary resistance, but did not significantly influence precapillary sphincters. The reactions were similar in skeletal muscle for NA and also for DHE, with the important exception that the latter drug usually elicited a moderate dilator response in the muscle resistance vessels.3. The average constrictor responses of the capacitance vessels were significantly larger for DHE than NA in skin and also in muscle despite the fact that DHE did not much affect the resistance vessels in muscle.4. The effects of DHE on the intestinal and renal vascular circuits in the cat were comparatively small. 5. Since the constrictor effect of DHE seems confined mainly to the capacitance vessels, the drug may have beneficial effects in circulatory disorders characterized by impaired venomotor regulation.A thorough description of the effects on the peripheral circulation of a vasoactive drug cannot be made unless quantitative information is available about the reactions evoked in the functionally differentiated consecutive sections of the vascular beds (see Mellander & Johansson, 1968). Drug induced changes of tone in the resistance vessels influence regional blood flow, and changes of activity in the precapillary 'sphincters' determine the number of patent capillaries and therefore, the size of the functional capillary surface area available for exchange.

The effects of static and dynamic passive stretch and shortening on electrical activity and active force were analyzed in the isolated rat portal vein. Static stretch by 40% of muscle length evoked moderate excitatory effects with enhanced mechanical activity and an average increase in spike discharge of 12% above the control value of 55 ± 2.6 spikes/min. The dynamic responses studied at various rates of length change (dL/dt) over the range between -12 and +12 mm/min, i.e., -3 and +3% muscle length/sec, were much more pronounced. Active force and spike activity showed graded increases with increasing rates of stretch. The electrical activity reached a value of 180 spikes/ min (r= 325% of control) at 5 mm/min; this frequency was then maintained for stretch rates up to 12 mm/min. Mechanical activity during stretch was further reinforced by the shift along the length-tension diagram. Passive shortening at rates from -1 to -12 mm/min caused graded decreases in mechanical and electrical activity below the control levels, complete inhibition being observed at the latter dL/dt. Blockade of a and /3 receptors indicated that the responses were myogenic in nature. The findings seem to provide direct support for the myogenic hypothesis of vascular tone and responses to stretch of the vascular wall, but they indicate that emphasis should be placed on the dynamic characteristics of the stimulus rather than its static nature. This emphasis constitutes a new concept in the myogenic control of the peripheral circulation.

LUNDVALL, J., S. MELLANDER, H. WESTLING and T. WHITE. Fluid transfer between blood and tissues during exercise. Acta physiol. scand. 1972. 85. 258-269.

The influence of nervous and local chemical factors on the pre‐capillary sphincter section of the muscle vascular bed has been studied in terms of changes in the filtration coefficient (CFC). The results suggest that the vasoconstrictor fibres, beside their powerful influence on the resistance and capacitance vessels, affect also the pre‐capillary sphincters, which determine the number of capillaries open to flow. However, the influence exerted by the centrally controlled vasoconstrictor fibres on the pre‐capillary sphincters in the skeletal muscles is rapidly overcome by ‘vasodilator metabolites’, accumulating when flow is reduced or metabolism is increased. Such local metabolic factors exert an especially powerful control of the sphincter section as compared with the neurogenic influence. Thus CFC in the skeletal muscles tends to rise at any decrease of blood flow/ tissue metabolism ratio e. g. from ‘resting’ values around 0.015 up to 0.04—0.05, during muscular exercise; an increase also occurs when flow is reduced. Flow reduction following mechanical arterial obstruction induces a relatively greater CFC increase than when flow is reduced by the constrictor fibres, which unmasks their weak but still significant influence on the pre‐capillary sphincter section.

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SUMMARY1. In anaesthetized cats, the hepatic artery, portal vein and inferior vena cava pressures and the hepatic artery and portal vein flows were recorded using pressure transducers and electro-magnetic flowmeters.2. The hepatic nerves were stimulated with maximal stimuli for periods of 2-5 min. The magnitude of the response varied with the frequency of stimulation over the range 1-10 impulses/sec. The resistance to flow increased in both the hepatic artery and the portal vein.3. In the hepatic artery, mean pressure remained virtually constant, while the flow showed an initial marked decrease followed by a return towards the control level. In the portal vein, the flow remained constant while portal pressure showed a maintained increase. These responses were unaffected by previous administration of atropine and propranolol, but were blocked by phenoxybenzamine.4. Infusions of noradrenaline into the hepatic artery produced changes similar to those following stimulation of the nerves. In contrast, when the hepatic arterial pressure was maintained constant, intravenous infusions of noradrenaline produced a maintained decrease in hepatic artery flow.5. The occurrence of autoregulation of the hepatic artery flow at arterial pressures above 80-100 mm Hg was confirmed.6. Occlusion of the carotid arteries caused a rise in arterial pressure with little change in hepatic artery flow, but when the hepatic artery pressure was maintained at the pre-occlusion level the flow showed an abrupt decrease, usually followed by a recovery towards the control level. This decrease was abolished by section of the hepatic nerves and removal of the adrenal glands.

MELLANDER, S., B. BERG and H. O n E L u M . Vascular adjustments to increased transmural pressure in cat and man with special reference to shifts in capillaiyjfuid transfer. Acta physiol. scand. 1964. 61. 34-48. -When a vascular bed is exposed to an increased transmural pressure, as occurs in the lower extremities in man on standing, adjustments of the vascular tone take place which tend to protect the organism against excessive transcapillary loss of circulating fluid into the extravascular space. Thus, first, by closure of a number of the precapillary sphincter vessels, the size of the capillary surface