Thoracic spinal neuron responses to repeated myocardial ischemia and epicardial bradykinin.

Bradykinin has been strongly implicated as a mediator of cardiac nociception. During coronary artery occlusion, the content of bradykinin in coronary sinus blood increases. In non-cardiac tissues nociception to bradykinin exhibits tachyphylaxis, however, this phenomenon has not been rigorously studied in the heart. This raises the question that repeated coronary occlusions may also result in tachyphylaxis, thereby reducing cardiac sensation on subsequent ischemic stimulation. We therefore examined the hypothesis that repetitive episodes of myocardial ischemia and of epicardial application of bradykinin demonstrate tachyphylaxis. Mongrel cats were anesthetized with alpha-chloralose and heart rate, arterial pressure, and thoracic spinal neuron firing rate were recorded during 60 s of anterior descending coronary occlusion or local epicardial application of bradykinin (10 microM). Neurons were identified by cutaneous receptive fields in the left shoulder area. Sixty-one of 93 neurons tested responded with an increase in firing rate to coronary artery occlusion only (n=24), bradykinin only (n=19) or to both (n=18). On repetitive coronary occlusion, 14 of 25 neurons demonstrated tachyphylaxis compared to 12 of 15 tested with bradykinin (p<0.05). Similar responses were observed in thoracic spinal neurons that projected to the brain. In neurons demonstrating tachyphylaxis, dorsal cervical cold block partially restored the neuronal activation to coronary occlusion but not to bradykinin. We conclude, based on neuronal responses to repetitive stimuli, that afferent spinal responses to coronary occlusion and bradykinin are different. These data suggest that bradykinin is not the sole mediator of myocardial ischemic pain. The tachyphylaxis to repeated coronary artery occlusions may contribute to the clinical phenomenon of silent myocardial ischemia.

Role of medullary lateral reticular formation in baroreflex coronary vasoconstriction.

We have recently identified a polysynaptic pathway traversing discrete regions of the hypothalamus, midbrain, and medulla, along which site-specific electrical and chemical activation produces coronary vasoconstriction as part of a sympathoexcitatory response. We tested for the potential functional significance of this pathway by examining the hypothesis that a medullary component is involved in carotid baroreflex induced coronary vasoconstriction. Coronary flow velocity was measured with a Doppler probe in anesthetized cats. Following vagotomy and propranolol, bilateral carotid occlusion produced an increase in mean arterial pressure (56 +/- 14%, means +/- S.E.M.) and in coronary vascular resistance (51 +/- 13%) which was greater than that (29 +/- 6%) expected from the concurrent rise in arterial pressure during aortic constriction. Bilateral microinjections of lidocaine into the medullary lateral reticular formation attenuated the reflex increase in pressure (11 +/- 2%) and virtually abolished the rise (8 +/- 2%) in coronary resistance. After one hour recovery, carotid occlusion again increased aortic pressure (56 +/- 13%) and coronary vascular resistance (47 +/- 15%). Microinjections of lidocaine outside this medullary region did not impair the coronary vasoconstrictor response to carotid occlusion. We conclude that the medullary lateral reticular formation contains neural elements which participate in baroreflex-induced changes in arterial pressure and coronary vascular resistance. Components of the previously described central coronary vasoconstrictor pathway may play a role in pathophysiological conditions associated with increased coronary vasomotor tone.

Tenascin-C in tendon regions subjected to compression.

Tendon regions subjected almost exclusively to tension differ from regions subjected to high levels of compression as well as tension. Regions not exposed to compression consist primarily of spindle-shaped fibroblasts surrounded by densely packed longitudinally oriented collagen fibrils formed principally from type-I collagen. In contrast, regions subjected to compression have a fibrocartilagenous structure and composition: they consist of spherical cells surrounded by a matrix containing hyaline cartilage proteoglycans (aggrecan) and type-II collagen as well as type-I collagen. Reducing their adhesion to the matrix may help cells in the latter regions establish and maintain a spherical shape and minimize their deformation when the tissue is subjected to mechanical stress. We hypothesized that expression of tenascin-C, an anti-adhesive protein, is part of the adaptation of tendon cells to compression that helps establish and maintain fibrocartilagenous regions. To test this hypothesis, we compared segments of bovine flexor tendons subjected to repetitive compression (distal) with segments that are not subjected to compression (proximal) to determine whether they differed in tenascin-C content and expression. RNA and protein analyses showed that tenascin-C expression was elevated in the distal tendon. Tendon cells from the distal segment expressed more tenascin-C mRNA than did cells from the proximal segments for as long as 4 days in cell culture, indicating that increased tenascin-C expression is a relatively stable feature of the distal cells. Moreover, purified tenascin-C inhibited the attachment of cultured tendon cells to fibronectin. These observations support the hypothesis that tenascin-C expression is a cellular adaptation to compression that helps establish and maintain fibrocartilagenous regions of tendons by decreasing cell-matrix adhesion.

The role of tenascin-C in adaptation of tendons to compressive loading.

Although most tendon regions are subjected primarily to high tensile loads, selected regions, primarily those that directly contact bones that change the direction of the tendon, must withstand high compressive loads as well. Compressed tendon regions differ from regions subjected to primarily tensile loads: they have a fibrocartilaginous structure with spherical cells surrounded by a matrix containing aggrecan and collagen types I and II, in contrast regions not exposed to compression have a fibrous structure with spindle shaped fibroblasts surrounded by a matrix of dense, longitudinally oriented type I collagen fibrils. The spherical shape of cells in fibrocartilagenous regions indicates these cells are more loosely attached to the matrix than their spindle-shaped counterparts in fibrous regions, a feature that may help to minimize cell deformation during tendon compression. We hypothesized that expression of tenascin-C, an anti-adhesive protein, is part of the adaptation of tendon cells to compression that helps establish and maintain fibrocartilaginous regions. To test this hypothesis we compared tenascin-C content and expression in compressed (distal) versus uncompressed (proximal) segments of bovine flexor tendons. Immunohistochemistry and immunoblot analyses showed that tenascin-C content was increased in the distal tendon where it co-distributed with type II collagen and aggrecan. Tendon cells from the distal segments expressed more tenascin-C than did cells from the proximal segments for up to four days in cell culture, indicating that increased tenascin-C expression is a relatively stable feature of the distal cells. These observations support the hypothesis that tenascin-C expression is a cellular adaptation to compression that helps establish and maintain fibrocartilagenous regions of tendons.