Dose-response curves were obtained from dog anterior tibial artery rings at various lengths (L) to determine whether sensitivity to norepinephrine (NE) and potassium (K+) depends on arterial circumference. The dose for half maximal response (ED50) was determined by graphical estimation and by calculation from a best fit curve. For both NE and K+: 1) ED50 was lowest (most sensitive) at L for maximum active force (Lmax) and increased significantly as L decreased from Lmax; 2) ED50 at 1.0 and 1.15 Lmax was not significantly different; 3) ED50 of repeated dose-response curves at Lmax was not significantly different; and 4) when the direction of length change was reversed (from decreasing to increasing), the direction of change in ED50 was also reversed (from increasing to decreasing). Change in the dose for 10% maximal response was the same as ED50. The results did not depend on the method of determining ED50 or on whether responses were expressed as absolute values or as relative values. The results show that sensitivity of vascular smooth muscle depends on L and that the length-sensitivity relation is similar to the length-active tension relation. Similarity of results for NE and K+ indicate that length-dependent sensitivity does not depend on the method of stimulation.
Previous work has shown that vascular smooth muscle sensitivity depends on muscle length (arterial circumference) at lengths equal to and less than that for maximum active force (Lmax). In the present study dose-response curves were obtained from dog anterior tibial artery rings at lengths equal to or longer than Lmax. The curves were compared with dose-response curves obtained at lengths less than Lmax. The agonist concentration for half maximal response (ED50) was determined by graphical estimation and by calculation from a best-fit curve. The results show that with norepinephrine (NE) stimulation 1) ED50 decreased significantly at each step when the rings were stretched from Lmax to 1.15 Lmax and then to 1.30 Lmax; 2) ED50 increased significantly when length was decreased from 1.15 to 1.00 Lmax; 3) ED50 decreased significantly at each step when the rings were stretched from 0.70 Lmax to Lmax and then to 1.30 Lmax; and 4) for NE concentration greater than the ED50 at Lmax, active stress was significantly higher at Lmax than at 0.70 Lmax or 1.30 Lmax. For an NE concentration less than the ED50 at Lmax, the active stress at 1.30 Lmax was higher than the active stress at Lmax and at 0.70 Lmax. The results show that sensitivity of vascular smooth muscle continually increases with stretch and does not have a maximum at the length for maximum active force.
Two species of Haplognathia (Gnathostomulida, Filospermoidea) were examined by electron microscopy and found to possess a tissue connection between the posterior end of the intestine and the dorsal epidermis. This anal tissue connection is characterized by a thinned out epidermis, interdigitation of epidermal and intestinal cells, and the absence of the basement membrane. An open anal pore was not observed. It is possible that the anal tissue connection of Haplognathia serves as a functional anal pore. The implications of this discovery with respect to the various theories of early Metazoan evolution are discussed.
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