Two-component arterial blood pressure conditional response in rat.

The objective of these experiments was to quantify the pattern of change in arterial blood pressure (BP) during a discriminative aversive classical conditioning paradigm in rat using a new "high resolution" computer analysis. Sprague-Dawley rats (n = 5) were restrained in a soft, conical cloth pouch and conditioned using a 6 sec. pulsed tone (CS+) followed by a 0.5 sec. tail shock; a steady tone, never followed by shock, served as a CS-. BP peaked at 16.4 +/- 6.5 mm Hg (mean +/- SD) above control at 1.5 +/- 0.1 sec. after onset of CS+. This "first component" ("C1") also occurred during CS- (12.1 +/- 3.8 mm Hg), although the magnitudes of the two were significantly (p < 0.05) different. Another group of rats (n = 8) was treated identically except the tones were 15 seconds long. The conditional BP response consisted of two components. C1 was reminiscent of that seen using the short tone: for CS+ a peak of 13.6 +/- 5.6 mm Hg at 1.5 sec. or, for CS-, of 10.0 +/- 4.3 at 1.3 sec. (p < 0.05). In CS+ trials BP peaked again ("C2," 7.4 +/- 2.5 mm Hg) at 8.3 +/- 1.2 sec. There was no statistically significant C2 for CS- trials, clearly demonstrating discrimination between tones. The unconditional BP response in both groups consisted of two large, closely spaced peaks in BP. Respiration was recorded in 3 additional rats. After shock delivery these subjects often showed a sudden shift between (1) a regular respiratory pattern with moderate chest excursion and (2) apneic episodes interspersed with single, deep breaths. This latter pattern was associated with large, low frequency fluctuations in BP. Continued development of the rat conditioning paradigm is especially warranted because of the ability to record sympathetic nerve activity in intact, awake subjects and the large number of readily available genetic strains, which model human pathological states.

Structure of myofibrils at extra-junctional membrane attachment sites in cultured cardiac muscle cells.

The composition and organization of myofibrils at extra-junctional membrane attachment sites in cultured neonatal rat cardiac muscle cells were analysed by immunofluorescence and electron microscopy. When myofibril terminals attached to the cell membrane via focal contacts at regions of the sarcolemma that lacked intercalated discs, they appeared to be non-striated and resembled thick actin cables. Although the non-striated terminals contained actin, myosin and alpha-actinin, the proteins were not organized into recognizable sarcomeres at the light microscopic level. Analysis of the structure of the terminals in the electron microscope confirmed that the usual sarcomeric organization and attachments to the sarcolemma were markedly modified. The non-striated myofibril terminals differed in structure from both stress fibres in non-muscle cells and stress fibre-like structures present in embryonic heart cells in culture. Non-striated myofibril terminals attached to the cell membrane by lateral contact with extra-junctional electron-dense membrane plaques rather than by insertion by their ends into the fascia adherens. It is proposed that the structure and composition of membrane-attachment points for myofibrils may have an influence on the structure, organization or stability of contractile elements in cardiac muscle.