Fertilization selection on egg and jelly-coat size in the sand dollar Dendraster excentricus.

Organisms with external fertilization are often sperm limited, and in echinoids, larger eggs have a higher probability of fertilization than smaller eggs. This difference is thought to be a result of the more frequent sperm-egg collisions experienced by larger targets. Here we report how two components of egg target size, the egg cell and jelly coat, contributed to fertilization success in a selection experiment. We used a cross-sectional analysis of correlated characters to estimate the selection gradients on egg and jelly-coat size in five replicate male pairs of the sand dollar Dendraster excentricus. Results indicated that eggs with larger cells and jelly coats were preferentially fertilized under sperm limitation in the laboratory. The selection gradients were an average of 922% steeper for egg than for jelly-coat size. The standardized selection gradients for egg and jelly-coat size were similar. Our results suggest that fertilization selection can act on both egg-cell and jelly-coat size but that an increase in egg-cell volume is much more likely to increase fertilization success than an equal change in jelly-coat volume. The strengths of the selection gradients were inversely related to the correlation of egg traits across replicate egg clutches. This result suggests the importance of replication in studies of selection of correlated characters.

Insulin reverses impaired acetylcholine-induced dilatation of the rat basilar artery during diabetes mellitus.

The goal of this study was to determine whether treatment of diabetic rats with insulin reverses impaired nitric oxide synthase-dependent reactivity of the basilar artery in vivo. We measured the diameter of the basilar artery in non-diabetic rats, diabetic (streptozotocin; 50-60 mg/kg i.p.) rats, and diabetic rats treated with insulin implants in response to acetylcholine and nitroglycerin before and following topical application of N(G)-monomethyl-L-arginine (L-NMMA). Reactivity of the basilar artery was measured 2-3 months after injection of streptozotocin. We found that topical application of acetylcholine (1.0 and 10 microM) produced dose-related dilatation of the basilar artery in non-diabetic rats, which was inhibited (>80%) by topical application of L-NMMA (10 microM). In diabetic rats, the magnitude of vasodilation produced by acetylcholine was significantly less than in non-diabetic rats. Further, topical application of L-NMMA did not affect dilatation of the basilar artery in diabetic rats in response to acetylcholine. In insulin treated diabetic rats, dilatation of the basilar artery in response to acetylcholine was significantly greater than that observed in either non-diabetic or diabetic rats. In contrast, dilatation of the basilar artery in response to nitroglycerin was similar in insulin treated diabetic rats, non-diabetic rats and diabetic rats. Topical application of L-NMMA only partially inhibited dilatation of the basilar artery in response to acetylcholine in diabetic rats treated with insulin. Thus, the findings of this study suggest that impaired acetylcholine-induced dilatation of the basilar artery during diabetes mellitus can be restored by treatment with insulin. Further, it appears that the contribution of cellular pathways, in addition to nitric oxide synthase, may contribute to dilatation of the basilar artery in response to acetylcholine in rats treated with insulin.

Constrictor responses of resistance arterioles during diabetes mellitus.

While diabetes mellitus appears to alter nitric oxide synthase-dependent vasodilatation, the effect of diabetes on constrictor responses of resistance arterioles is not clear. Our goal was to examine the effect of diabetes on constrictor responses of cheek pouch arterioles. In vivo diameter of arterioles ( approximately 50 microm) was measured in response to norepinephrine, the thromboxane analogue (U-46619) and endothelin-1 in nondiabetic and diabetic hamsters (4-6 weeks post streptozotocin). Norepinephrine (1.0 and 10 nM) and U-46619 (0.1 and 1.0 nM) produced similar dose-related vasoconstriction in nondiabetic and diabetic hamsters (P > 0.05). In contrast, vasoconstriction to endothelin-1 (0.1 and 1.0 pM) was greater in diabetic than nondiabetic hamsters (P < 0.05). Next, we examined the role of nitric oxide in basal vascular tone and whether enhanced vasoconstriction in diabetic hamsters to endothelin-1 might be related to an alteration in the modulatory role of nitric oxide. N(G)-monomethyl-L-arginine (L-NMMA) (1.0, 10 and 100 microM) produced dose-related vasoconstriction in nondiabetic, but not diabetic hamsters. Further, L-NMMA did not alter vasoconstriction in response to endothelin-1 in nondiabetic and diabetic hamsters. These findings suggest that diabetes alters constriction of cheek pouch resistance arterioles to endothelin-1 which appears to be independent of the synthesis/release of nitric oxide. In addition, based upon findings using L-NMMA, it appears that there is a reduced influence of nitric oxide on basal diameter of resistance arterioles during diabetes.