The small arteries play an important functional role in establishing the increased peripheral resistance found in essential hypertension. This paper concerns the direct measurement of the intrinsic mechanical and contractile properites of two categories of small arterial resistance vessels in the mesenteric bed of 5-month-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The vessels had mean internal diameters of 246 fim and 153 /*m when relaxed at 100 mm Hg effective transmural pressure. Segments (1 mm) were mounted in such a way that internal circumference could be controlled and the circumferential wall tension (T) measured. After mounting, each vessel was maximally activated (by K* depolarization at 37°C in the presence of 5 mM Ca) at an internal circumference for which AT was approximately maximal, where AT = Tactlve Trelaxed. From the average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessel would average values of AT measured we have estimated (on the basis of Laplace's equation) that the SHR vessels would have been able to contract against 34% greater pressures than the WKY vessels (P < 0.001). Optical measurements of the dimensions of these vessels showed a 23% greater wall thickness in the SHR vessels (P < 0.02). There were no significant differences in the calculated active wall stresses of the SHR and WKY vessels; this suggests that the greater contractility found in the SHR vessels may be due to their having a greater smooth muscle cell content. These vessel measurements have been examined as well as the rats' blood pressures and heart to body weight ratios. The comparison points to the possibility that the disturbance to the cardiovascular regulatory system which results in hypertension produces similar cellular responses in both the myocardium and the peripheral vasculature.

Contractile Properties of Small Arterial Resistance Vessels in Spontaneously Hypertensive and Normotensive Rats

Contraction of smooth muscle is regulated by the cytosolic Ca2+ level ([Ca2+]i)b, and the sensitivity to Ca2+ of the contractile elements in response to changes in the environment surrounding the cell. The first sequence of events in regulation includes the binding of endogenous substances, such as

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Calcium Movements, Distribution, and Functions in Smooth Muscle

The role of a transmembrane calcium influx in hypoxic pulmonary vasoconstriction was studied in isolated, blood-perfused, rat lungs. We reasoned that, if the influx of extracellular calcium mediated the hypoxic mechanism, pressor responses to alveolar hypoxia (2.5% O2) would be susceptible to inhibition by the calcium antagonists verapamil (2 X 10(-5) to 2 X 10(-1) mM) and SKF 525A (2.6 to 260 mM). Susceptibility of hypoxic pressor responses to inhibition by these calcium antagonists was contrasted to that of pressor responses elicited by the humoral vasoconstrictors angiotensin II(1 or 0.5 mug) and prostaglandin F2alpha (10 myg). Since neither saralasin (0.5 muM), a competitive antagonist of angiotensin II, nor meclofenamate (6.8 muM), an inhibitor of prostaglandin synthesis, depressed hypoxic pressor responses, it was concluded that these humoral transmitters were not directly involved in the hypoxic mechanism, and therefore served as independent reference agonists. The order of susceptibility of pulmonary pressor responses to inhibition by verapamil was hypoxia greater than angiotensin II greater than prostaglandin F2alpha. SKF 525A also reduced pressor responses to hypoxia more readily than those to angiotensin II. The greater inhibition of hypoxic pulmonary vasoconstriction by both calcium antagonists suggested that the hypoxic mechanism was critically dependent on the transmembrane influx of extracellular calcium. Mediation of the hypoxic response by this type of excitation-contraction coupling is consistent with the idea that hypoxia has a direct depolarizing effect on the vascular smooth muscle. It also provides a unifying explanation for inhibition of the hypoxic mechanism by various agents that have depressant or stabilizing actions on membranes in addition to other pharmacological effects.

Inhibition of hypoxic pulmonary vasoconstriction by calcium antagonists in isolated rat lungs.

Excitation-contraction coupling in smooth and cardiac muscle can be dis­ rupted by a number of organic compounds known collectively as Ca antago­ nists (CAts). In 1964, Fleckenstein (1) provided the first important clue regarding their possible mechanism of action when he noted that the cardiac effects of two of these agents, verapamil and prenylamine, were similar to those of Ca2+ removal from the extracellular space. A few years later, he proposed that CAts exerted their negative inotropic and coronary vasodila­ tor effects by blocking the entry of Ca2+ into the cells (2, 3). The vasodila­ tor and cardiac effects of CAts are having a great impact on cardiovascular medicine; consequently many reviews and monographs on this subject have appeared in the recent past (4--9). The purpose of this review is to provide an in-depth analysis of the mechanisms of action of CAts, rather than to compile an exhaustive survey of the literature. Only when we know how CAts interact with the processes that regulate Ca availability and sensitivity in smooth muscle can we understand why agents that "antagonize" the action of such a universal intracellular messenger can be effectively de­ ployed as selective therapeutic agents in cardiovascular medicine.

Mechanisms of calcium antagonist-induced vasodilation.

1. The amplitude of the noradrenaline-sensitive Ca stores has been estimated by measuring the amplitude of the transient contraction induced by the agonist in Ca-free solution. or by measuring the amount of 45Ca released under these conditions. 2. The rate of filling of this store after depletion is much faster than the rate of depletion in Ca-free solution, and depends on [Ca]o in the bathing solution. The degree of filling also depends on [Ca]o. 3. At the same [Ca]o the degree of filling is higher in K-depolarized tissues than in control tissues. However at 10 mM-[Ca]o and 5.9 mM-K the amount of Ca taken up by the store is larger than that after loading in 0.2 mM-Ca and 141.4 mM-K, although the tissues remain relaxed during loading at 5.9 mM-K and contracted at 141.4 mM-K. 4. The Ca antagonists D600 and nicardipine selectively block the contraction induced by K depolarization, but do not affect appreciably the noradrenaline-induced contraction. 5. The filling of the store is not significantly reduced by the presence of the Ca antagonists in solutions containing 5.9 mM-K. However these antagonists reduce the degree of filling in K-rich loading solution to a level which is lower than that observed in the control. 6. Mn blocks both the contraction induced by K-rich solution and the tonic component of the noradrenaline-induced contraction and its also inhibits filling of the store. 7. The results suggest that the filling of the store under physiological conditions occurs by a direct pathway between the store and the extracellular medium.

Exchange characteristics of the noradrenaline-sensitive calcium store in vascular smooth muscle cells or rabbit ear artery.

1.

Contraction of helical strips of the rat aorta was induced by 3 μg/ml of noradrenaline or by potassium-rich solution (80–120 mM K). The resulting amplitudes of contraction were maximal for these respective ways of activation and showed almost the same value in this preparation.




2.

The noradrenaline-induced tension, but not the contraction induced by depolarization, decreased, when the hydrogen ion concentration of the bath solution was increased. Furthermore, the response to noradrenaline was more sensitive to a blockade of the adrenergic alpha-receptors or a variation of the bath temperature.




3.

Since, after the application of papaverine, the amplitudes of contraction produced by noradrenaline or depolarization were diminished to the same degree, papaverine probably acts on the later steps of the activation-contraction mechanism.




4.

When verapamil (iproveratril) was applied in order to inhibit excitationcontraction coupling, tension development after depolarization was influenced to a higher degree than noradrenaline-induced contraction.




5.

The fact that it is possible to inhibit the response to depolarization without substantial influence on the response to noradrenaline andvice versa led to the conclusion that in the rat aorta depolarization on the one hand and noradrenaline on the other hand act via different mechanisms of activation.

Activation of vascular smooth muscle of rat aorta by noradrenaline and depolarization: Two different mechanisms

An isolierten Spiralstreifen der Rattenaorta wurde der Einflus der Badtemperatur auf die passive Ruhespannung und die maximale, durch Noradrenalin (=NA) bzw. Kalium (=K) induzierte Kontraktionsamplitude untersucht.

1.

Zwischen 37°C und 15°C verminderte eine Abkuhlung den Basaltonus reversibel um jeweils 20 dyn/°C (Abb.1a-d und 4a).




2.

Die adrenerg ausgeloste Kontraktionsamplitude besas ein Optimum um 37°C, war unter 33,5°C linear zur Temperatur korreliert und bei 15°C nahezu aufgehoben (Abb.3a). In Ca-armer Losung (1,1 statt 2,4 mM/l) zeigte sich ein signifikant starkerer Temperatureinflus (Tab.1).




3.

Bei 37°C betrug die zur maximalen adrenergen Aktivierung erforderliche NA-Konzentration 2,4·10−6 g/ml. Sie sank zwischen 37°C und 25°C exponentiell um mehr als eine Zehnerpotenz.




4.

Die durch 136 mM/l K erzeugte Spannung zeigte nur eine geringe Temperaturabhangigkeit (Abb.4b). Eine Senkung der Ca-Konzentration blieb dabei ohne signifikanten Einflus (Tab.1).




5.

Bei einem Temperatursprung von 20°C auf 30°C ergab sich fur die adrenerge Aktivierung ein Q10 von 2,8 und fur die Depolarisation einQ10 von 1,23. Dieser Befund spricht in Ubereinstimmung mit fruheren Ergebnissen fur eine unterschiedliche Aktivierung der kontraktilen Elemente.

[Differential temperature dependency of vascular smooth muscle contraction following activation by potassium-depolarization or noradrenaline].

The length-tension relationships of resting or activated (by 130 mM of potassium) helical strips of the pig coronary artery were studied. The distensibility of the preparations was expressed by approximated exponential function. In non-activated strips, an average lengthening of 9.64% was necessary for doubling the resting tension. The maximum of active tension (isometric contractions) occurred at high degrees of muscle stretch, whereas the maximum of shortening (isotonic free-loaded contractions) was found at lower values of resting tension. If the theory of the sliding mechanism is a correct assumption for the contraction process of vascular smooth muscle, the maximum of active tension observed at a muscle stretch of about 8000 dynes/mm2 is obviously caused by an optimal overlapping of actin and myosin filaments. At this high degree of muscle stretch, however, a great number of cross-linkages is necessary to overcome the passive tension and only few cross-linkages are available for shortening. Therefore, in isotonic contractions the amount of shortening is diminished and the time to peak of contraction is augmented with elevated resting tension exceeding 1000 dynes/mm2.

Length-tension relationships in resting and activated vascular smooth muscle fibres

1.

Determinations of the maximum tension development of the helical strip of rat aorta showed that the maximal response to vasopressin amounted to 75% and that to potassium-induced depolarization to 93% of the contraction caused by noradrenaline (=100%).




2.

When phentolamine (1.2-6.2×10−6 g/ml) was added to the bath 15 min before the determination of dose-response curves for noradrenaline, there was a concentration-dependent shift of the curve to the right; the slope of the curve, the ED50 and the concentration of noradrenaline required for maximum activation of the muscle increased. However, when phentolamine (1.2 to 6.2×10−6 g/ml) was administered after the development of a maximum response to noradrenaline, the muscle relaxed nearly completely. In the presence of phenoxybenzamine (3×10−6 g/ml) noradrenaline failed to cause contractions.




3.

The dose-response curve for vasopressin was sigmoid. The presence of phentolamine did not affect the ED50 (at 1.6×10−3 IU/ml) or the peak of the curve (at 1.2×10−2 IU/ml). The administration of vasopressin to depolarized muscles always caused a contractile response. When the muscle was activated repeatedly by vasopressin, tachyphylaxis occurred.




4.

Block of the adrenergic alpha-receptors influenced the contractile response to depolarization far less than that to noradrenaline.




5.

In accordance with earlier studies, the present results lead to the conclusion that the contraction of the vascular smooth muscle in response to noradrenaline, vasopressin, or depolarization, respectively, involves different mechanisms.

Aktivierung des isolierten Gefäßmuskelpräparates durch Noradrenalin, Vasopressin bzw. Depolarisation bei stufenweiser Blockade der alpha-Receptoren

The contraction of the vascular smooth muscle is induced by releasing calcium ions into the cell. After depolarization of the membrane the calcium-influx from the extracellular space increases. This mechanism of activating the contraction is called the electromechanical coupling. By stepwise elevation of the extracellular potassium concentration it is possible to activate this mechanism gradually (1, 2).

L. Griebel

Papers

Activation of vascular smooth muscle of rat aorta by noradrenaline and depolarization: Two different mechanisms

[Differential temperature dependency of vascular smooth muscle contraction following activation by potassium-depolarization or noradrenaline].

Length-tension relationships in resting and activated vascular smooth muscle fibres

Aktivierung des isolierten Gefäßmuskelpräparates durch Noradrenalin, Vasopressin bzw. Depolarisation bei stufenweiser Blockade der alpha-Receptoren

Different Mechanisms of Activating the Vascular Smooth Muscle by Noradrenaline or Potassium-Depolarization