No abstract

A broad class of generalized linear mixed models, e.g. variance components models for binary data, percentages or count data, will be introduced by incorporating additional random effects into the linear predictor of a generalized linear model structure. Parameters are estimated by a combination of quasi‐likelihood and iterated MINQUE (minimum norm quadratic unbiased estimation), the latter being numerically equivalent to REML (restricted, or residual, maximum likelihood). First, conditional upon the additional random effects, observations on a working variable and weights are derived by quasi‐likelihood, using iteratively re‐weighted least squares. Second, a linear mixed model is fitted to the working variable, employing the weights for the residual error terms, by iterated MINQUE. The latter may be regarded as a least squares procedure applied to squared and product terms of error contrasts derived from the working variable. No full distributional assumptions are needed for estimation. The model may be fitted with standardly available software for weighted regression and REML.

Chronic pressure or volume overload can cause the vertebrate heart to remodel. The hearts of fish remodel in response to seasonal temperature change. Here we focus on the passive properties of the fish heart. Building upon our previous work on thermal-remodeling of the rainbow trout ventricle, we hypothesized that chronic cooling would initiate fibrotic cardiac remodeling, with increased myocardial stiffness, similar to that seen with pathological hypertrophy in mammals. We hypothesized that, in contrast to pathological hypertrophy in mammals, the remodeling response in fish would be plastic and the opposite response would occur following chronic warming. Rainbow trout held at 10°C (control group) were chronically (>8 weeks) exposed to cooling (5°C) or warming (18°C). Chronic cold induced hypertrophy in the highly trabeculated inner layer of the fish heart, with a 41% increase in myocyte bundle cross-sectional area, and an up-regulation of hypertrophic marker genes. Cold acclimation also increased collagen deposition by 1.7-fold and caused an up-regulation of collagen promoting genes. In contrast, chronic warming reduced myocyte bundle cross-sectional area, expression of hypertrophic markers and collagen deposition. Functionally, the cold-induced fibrosis and hypertrophy were associated with increased passive stiffness of the whole ventricle and with increased micromechanical stiffness of tissue sections. The opposite occurred with chronic warming. These findings suggest chronic cooling in the trout heart invokes a hypertrophic phenotype with increased cardiac stiffness and fibrosis that are associated with pathological hypertrophy in the mammalian heart. The loss of collagen and increased compliance following warming is particularly interesting as it suggests fibrosis may oscillate seasonally in the fish heart, revealing a more dynamic nature than the fibrosis associated with dysfunction in mammals.

Thermal acclimation causes the heart of some fish species to undergo significant remodelling. This includes changes in electrical activity, energy utilization and structural properties at the gross and molecular level of organization. The purpose of this Review is to summarize the current state of knowledge of temperature-induced structural remodelling in the fish ventricle across different levels of biological organization, and to examine how such changes result in the modification of the functional properties of the heart. The structural remodelling response is thought to be responsible for changes in cardiac stiffness, the Ca2+ sensitivity of force generation and the rate of force generation by the heart. Such changes to both active and passive properties help to compensate for the loss of cardiac function caused by a decrease in physiological temperature. Hence, temperature-induced cardiac remodelling is common in fish that remain active following seasonal decreases in temperature. This Review is organized around the ventricular phases of the cardiac cycle – specifically diastolic filling, isovolumic pressure generation and ejection – so that the consequences of remodelling can be fully described. We also compare the thermal acclimation-associated modifications of the fish ventricle with those seen in the mammalian ventricle in response to cardiac pathologies and exercise. Finally, we consider how the plasticity of the fish heart may be relevant to survival in a climate change context, where seasonal temperature changes could become more extreme and variable.