Modulation of atrial contraction by PKA and PKC during the compensated phase of eccentric cardiac hypertrophy.

Calcium homeostasis is intimately regulated by protein kinase phosphorylation cascades that are also involved in the induction and maintenance of cardiac hypertrophy. In addition, the development of cardiac hypertrophy has been associated with alteration in the activation of the adrenergic system. Therefore, we investigated the specific role of protein kinase A (PKA) and C (PKC) on cardiac muscle contractile activity in the presence and absence of adrenergic stimulation. Isolated left atrial preparations from sham- and volume overload-induced cardiac hypertrophied rats were superfused with Tyrode and electrically stimulated at 0.75 Hz. Contraction was assessed in the basal and pre-stimulated (norepinephrine, 10(-9)M) states. Specific inhibitors, KT 5720 for PKA and Ro-32-0432 for PKC, were used. Peak tension development in left atria from sham-operated rats was more sensitive to PKC- than PKA-inhibition, whereas this differential sensitivity was abolished in the hypertrophied hearts. This difference was mainly due to an increase in the role of PKA in the contractile response. Developed peak tension by left atria from shunt rats was higher than that from sham rats, but when expressed to relative tissue mass, hypertrophied muscle showed weaker contraction than that from the sham group. In addition, the left atrial velocity of contraction in the sham is PKA-sensitive, while that of the shunt is PKC-sensitive. Furthermore, the velocity of relaxation shows dependency on both protein kinases, with PKC having a greater effect than PKA in the hypertrophied group. NE increased the PTD and the velocity of contraction (+dT/dt) through PKA and PKC dependent mechanisms, without affecting the velocity of relaxation (-dT/dt) in atrial muscle from sham rats. In contrast, during eccentric hypertrophy NE effectively reduced PTD as well as the -dT/dt through a PKC-dependent mechanism. The present study demonstrates that during early development of moderate eccentric cardiac hypertrophy there is: (1) a reduced specific peak tension developed due to an imbalance in the PKA and PKC activation; (2) a change in the protein kinase dependence of the velocity of contraction and relaxation from PKA to PKC with atrial hypertrophy; and (3) a negative inotropic response to adrenergic receptor stimulation. These functional responses may play a critical role in the cardiac performance during the progression of eccentric cardiac hypertrophy into the decompensated phase and heart failure.

Paraventricular oxytocin neurons are involved in neural modulation of breathing.

In this study, we determined the projections of oxytocin-containing neurons of the paraventricular nucleus (PVN) to phrenic nuclei and to the rostral ventrolateral medullary (RVLM) region, which is known to be involved in respiratory rhythm generation. Studies were also designed to determine oxytocin-receptor expression within the RVLM and the physiological effects of their activation on respiratory drive and arterial blood pressure. Oxytocin immunohistochemistry combined with cholera toxin B, a retrograde tracer, showed that a subpopulation of oxytocin-containing parvocellular neurons in the dorsal and medial ventral regions of the PVN projects to phrenic nuclei. Similarly, a subpopulation of pseudorabies virus-labeled neurons in the PVN coexpressed oxytocin after injection of pseudorabies virus, a transynaptic retrograde marker, into the costal region of the diaphragm. A subpopulation of oxytocin expressing neurons was also found to project to the RVLM. Activation of this site by microinjection of oxytocin into the RVLM (0.2 nmol/200 nl) significantly increased diaphragm electromyographic activity and frequency discharge (P < 0.05). In addition, oxytocin increased blood pressure and heart rate (P < 0.05). These data indicate that oxytocin participates in the regulation of respiratory and cardiovascular activity, partly via projections to the RVLM and phrenic nuclei.

Adrenergic-mediated effects of cocaine on the myocardial force-frequency relationship.

We studied the role of sarcolemmal alpha- and beta-adrenoceptors activation in the effects of cocaine on the positive force staircase in isolated guinea pig atria. The preparations were superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (Vmax T) and time to peak tension (TPT). The positive force staircase was not affected by propranolol or phentolamine, but it was abolished by nifedipine. Cocaine 1 mg/l (2.9 microM) enhanced PTD and Vmax T, while TPT remained unchanged. On the other hand, cocaine did not modify the increase in PTD induced by the increase in frequency of stimulation, but significantly reduced the magnitude of the increase in Vmax T. The cocaine-induced attenuation of the increase in Vmax T in response to changes in the frequency of stimulation was abolished by both propranolol and phentolamine. It is concluded that the effect of cocaine on the force-frequency relationship required background activation of alpha- and beta-adrenergic receptors.

Mechanism of the positive inotropic action of cocaine in the guinea pig atrium.

We studied the mechanism of the positive inotropic action of cocaine in isolated guinea pig atria superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed, maximum velocity of development of tension, and time to peak tension. Cocaine 2.9 microM enhanced peak tension developed and velocity of development of tension, and prolonged time to peak tension. The increase in peak tension developed produced by cocaine was not affected by propranolol. On the other hand, the cocaine-induced increase in velocity of development of tension was reduced, but not abolished. In the presence of propranolol and phentolamine combined, the cocaine-induced prolongation of time to peak tension was abolished and the increases of both peak tension developed and velocity of development of tension were significantly smaller than those observed in the absence of the two adrenergic blockers. For all practical purposes, nifedipine completely abolished the increase in peak tension developed induced by cocaine. It is concluded that the positive inotropic effect of cocaine in the guinea pig atrial muscle is predominantly the result of adrenergic-dependent, both alpha- and beta- receptor mediated, as well as adrenergic-independent increases in calcium influx through the L-type calcium channels in the sarcolemma.

Mechanism of the negative inotropic action of cocaine in rat papillary muscle.

We studied the mechanism of the negative inotropic action of cocaine in isolated rat papillary muscles superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (Vmax T), and time to peak tension (TPT). The results show that in preparations driven at 1 Hz, 5 and 10 mg/liter cocaine had a significant negative effect only on PTD, while 20 mg/liter (58 microM) significantly depressed all three measured parameters. Similar results were obtained in preparations driven at 0.5 Hz, which developed a greater control active tension than those driven at 1 Hz. In preparations driven at 0.5 Hz, propranolol (1 microM) had no significant effect on the active tension developed nor did it modify the negative inotropic action of cocaine. Both nifedipine (1 microM) and caffeine (1.5 mM), on the other hand, significantly reduced PTD, Vmax T, and TPT. The depressant inotropic action of cocaine was accentuated by nifedipine, but was blocked by caffeine. These observations suggest that the primary site of action of cocaine was at the level of the sarcoplasmic reticulum rather than at the sarcolemma.

Distensibility of small pulmonary blood vessels.

Although vasomotor activity in small pulmonary vessels has been studied extensively in the past, using the concept of resistance to flow, information on the distensibility of these vessels is very sparse. In an attempt to reduce this deficit, we adapted a theoretical method developed for small systemic vessels, to estimate distensibility of pulmonary resistance vessels in experimental animals and man. Pressure-flow data from 11 dogs and 10 human subjects (5 control subjects and 5 patients with long-standing left heart failure) were used to calculate distensibility of small pulmonary vessels. The conductance, G, was calculated from these data as the ratio of blood flow to driving pressure. The slope of the relationship between the logarithm of G1/4 and the average distending pressure (ADP) provides a graphic picture of circumferential extensibility, E, defined as percent change in radius for an infinitesimal change in ADP. Results indicate that: (1) the value of E in dogs was 1.85 +/- .40 mmHg-1 for the control state, which decreased to 1.45 +/- .43 mmHg-1 during norepinephrine administration; however, the decrease in the value was not statistically significant (p = 0.53); (2) the value of E in control human subjects was 3.38 +/- .47 mmHg-1 and the value of E in patients with left heart failure was -0.64 +/- 0.39 mmHg-1; the difference was significant (P = .0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Cocaine enhances postrest and paired-stimulation potentiation in rat papillary muscle.

We studied the effects of cocaine (20 mg/l; 58 microM) on paired-stimulation potentiation and rest-potentiation in isolated rat papillary muscles superfused with Tyrode's solution at 31 degrees C. A force transducer was used to measure peak tension developed, maximum velocity of development of tension and time to peak tension. The results show that, in preparations driven at a basic constant rate (0.5 Hz), cocaine had a depressant effect on peak tension developed and maximum velocity of development of tension and shortened time to peak tension. Propranolol or nifedipine did not modify the negative inotropic action of cocaine, whereas this action was blocked by caffeine. The results also show that cocaine enhanced rest-potentiation and paired-stimulation potentiation. In the presence of propranolol, the enhancement of paired-stimulation potentiation caused by cocaine remained intact, whereas it failed to do so under the influence of nifedipine. On the other hand, nifedipine did not affect the enhancing action of cocaine on rest-potentiation. Caffeine reversed both rest-potentiation and paired-stimulation potentiation; this action was not modified by cocaine. Thus, even though cocaine exerted a negative inotropic action on rat papillary muscles driven at a constant rate, it enhanced the potentiating action of maneuvers which modify the force-interval relation. Cocaine appears to cause these effects by acting at various levels of electro-mechanical coupling, such as the sarcolemma, the sarcoplasmic reticulum and the myofilaments sensitivity to calcium.

Role of muscarinic receptors in renal response to acetylcholine.

Renal arterial infusion of acetylcholine (ACh) (40 micrograms/min) in control dogs produced an ipsilateral increase in renal plasma flow (RPF) and in sodium excretion (UNaV) without a change in glomerular filtration rate (GFR). The increase in RPF and UNaV was maintained during the infusion of ACh. In indomethacin (Indo)-treated dogs (5 mg/kg) ACh produced a transient rise in RPF and UNaV, followed by a progressive decline in RPF and UNaV. The profound renal vasoconstriction was accompanied by a decline in GFR. To determine the role of the muscarinic receptor in the renal vasodilation and in vasoconstriction produced by ACh in Indo-treated dogs, atropine at 6, 60, and 600 micrograms/min was infused into the renal artery before and during the infusion of ACh. In Indo-treated dogs, all dosages of atropine prevented renal vasoconstriction by ACh. Renal arterial infusion of atropine at 600 micrograms/min completely inhibited the renal vasodilation produced by ACh. Atropine infused at 60 micrograms/min partially inhibited, whereas 6 micrograms/min atropine failed to inhibit, the renal vasodilation produced by ACh. Our data suggest that the renal vasodilator and vasoconstrictor effects of ACh in Indo-treated dogs are mediated by two separate types of muscarinic receptors.

Clinical relevance of distal arterial compliance.

AIM: To calculate the compliance of resistance vessels. METHODS: Pressure-flow data (plethysmographic and sphygmomanometric) were obtained non-invasively from six normal and six hypertensive subjects, and the results were compared with similar data obtained previously from large blood vessels. The parameter used to represent compliance was extensibility (E), defined as the percentage change in radius for a given change in pressure. RESULTS: The hand vessels of hypertensive subjects (E = 0.126 +/- 0.034/mmHg) were significantly stiffer (P < 0.02) than those of the normotensive subjects (E = 0.272 +/- 0.047/mmHg); and the values of E for the resistance vessels were larger than those for the large arteries.

Effect of hypertension on elasticity and geometry of aortic tissue from dogs.

Inflation-extension experiments were carried out on segments of the descending thoracic aortas from 4 normotensive and 4 hypertensive dogs rendered hypertensive using either unilateral or bilateral renal artery constriction. Intravascular pressures up to 200 mm Hg and axial forces up to 200 g were used. The external diameter of the segment and the distance between two longitudinally spaced gage marks were recorded photographically at each pressure-force level combination. Dimensions in the underformed configuration were measured at the end of the inflation-extension experiment. Data were analyzed for changes in geometry and force-deformation response. Results indicate that: 1. Under sustained hypertension the wall thickness in the underformed configuration increases with a concurrent reduction in the in-situ longitudinal extension ratio. 2. This dual tissue response accomplishes substantial reductions in the circumferential and longitudinal stresses from the levels that would be reached at equivalent pressures in the absence of these geometric changes. 3. At comparable intravascular pressures the extensibility in the circumferential direction is slightly greater for the hypertensive aortas as compared to normals. However, the stress-extension ratio relationship in the circumferential direction is similar in the two groups. 4. The stress-extension ratio relationship in the longitudinal direction indicates that the hypertensive aorta is stiffer than its normotensive counterpart.

A theoretical method for estimating small vessel distensibility in humans.

A simple theoretical approach is presented for estimating vascular distensibility of small blood vessels from noninvasively obtained pressure-flow data in the hand and forearm of human subjects. To the extent that Poiseuille's law applies to blood flow in these vascular beds, conductance (the reciprocal of vascular resistance) can be calculated from these data as the ratio of blood flow to mean arterial pressure. The fourth root of the conductance is proportional to the radius of the vascular bed. The slope of the relation between the logarithm of the radius of the vascular bed and the transmural pressure is proportional to the vascular extensibility (E), which, in turn, for small deformations and constant vascular length, is proportional to the distensibility of small blood vessel. Data obtained from the hands of six hypertensive subjects were compared with that obtained from six normotensive subjects, all with their vascular beds in a maximally dilated state. Also compared were data obtained from four normal subjects with their vascular beds in the resting state and when the beds were maximally dilated. The results indicate that 1) in the hypertensive subjects, the small blood vessels of the maximally dilated vascular bed of the hand are significantly (p less than 0.02) less distensible (E = 0.126 +/- 0.034/mm Hg) than those in the normotensive subjects (E = 0.272 +/- 0.047/mm Hg) and 2) the small blood vessels of the normal forearm at resting levels of vasomotor tone are more distensible (E = 1.00 +/- 0.38/mm Hg) than in the maximally dilated state (E = 0.51 +/- 0.08/mm Hg).

Absence of increase in left ventricular mass in dogs with bilateral renovascular hypertension.

Using sonomicrometric and indwelling pressure transducers, 16 healthy, normotensive dogs were prepared for semicontinuous monitoring of left ventricular (LV) mass, volumes, pressures, flows and wall tension (W Ten), aortic pressure (AP), total peripheral resistance (TPR), and indices of myocardial contractility. After a period of control measurements, bilateral renal artery constriction (2K-2C) was produced in 5 dogs and monitoring continued for 3 weeks or more. Mean arterial pressure (MAP) and LV end-systolic pressure (LVESP) was significantly higher (P less than 0.01) in the 2K-2C dogs compared to controls. There was no significant difference in LV mass, W Ten, or dP/dt, although TPR was 65% higher in the 2K-2C dogs. Comparable data from five dogs with single kidney hypertension (1K-1C), after 3 weeks, showed an elevated MAP and LVESP similar to the 2K-2C dogs. This group also showed a significant increase in LV mass and dP/dt (P less than 0.01), while the TPR was significantly decreased. The data suggest that the mechanism of hypertension development in the 2K-2C dog is different from that of the 1K-1C dog, and that there is evidence of increase in neuroadrenergic activity (viz, myocardial contractility) associated with the development of LV hypertrophy. The role of W Ten in the hypertrophic process was less clear from these data. Further study of these models may provide insight into the relative roles of the neuroadrenergic and volume-vasoconstriction components in the pathogenesis of hypertension and changes in LV mass.

Noninvasive determination of small vessel distensibility in man.

The present study describes and illustrates a noninvasive method for computing extensibility of the 'resistance vessels', from an experimentally obtained resistance vs transmural pressure plot, in a maximally dilated vascular bed of man. Limb blood flow was measured plethysmographically following 10 min of arterial occlusion, and blood pressure was measured by the arm cuff method. A ratio of two resistance values taken 30 mm Hg apart was used to compute percent change in radius and the extensibility, E. The computed values of E were 0.71 mm Hg-1 for normal hand and 0.13, 0.22 and 0.69 for the hypertensive forearm, hand and finger respectively. These results indicate that the method may prove useful in the understanding and management of diseases such as hypertension or other small vessel diseases.