1.1 levels (http://www.luontoportti.com/suomi/en/kalat/roach). Therefore, roach may play

1.1    Roach an Ectothermic fish

 

Fish species
have ability to adapt and distribute in differnt aquatic habitats that extend
from freshwater to marine water, from cold polar seas to warm tropical reefs,
and from shallow surface waters to the intense pressures of the ocean depths.
To this fact, unsurprisingly that fishes represent the most variable and
largest group among vertebrates reached to more than 33000 recorded species (http://fishbase.org/home.htm; Powers,
1989; Axelsson et al., 1992; Johnston et al., 1994). Excluding 30 endothermic
species, all fishes are ectotherms where body temperaure was consistent with
the ambient water temperture (Dickson and Graham, 2004). Adaptaion strategy of
fish species is habitat and life style dependent by making physiological
adjustments. Some fishes are dynamic in winter and relatively inactive in
summer (e.g. burbot, Lota lota) (Edsall et al., 1993; Carl et
al., 1995; Pääkkönen and Marjomäki, 2000), others are active in summer and
dormant in winter (e.g. crucian carp, Carassius carassuis) (Holopainen et al., 1997), Others
are active all over the year (e.g. rainbow trout, Oncorhynchus mykiss). Their
ability to adapt to different environments make them a distinguished model to
investigate the organismic and molecular adjustments for surviving.

Roach (Rutilus rutilus; Linnaeus,
1758) is a cyprinid fish that inhabits fresh and brackish water in most of
Europe, except the Mediterranean zone, and western Asia. In Finland, Roach are the
most distributed fishes after perch and pike along all Finland’s coast and in
freshwater throughout the country with the exception of northernmost Lapland. It
is adaptable fish but does not tolerate acidity. Favours eutrophic water bodies
and numbers have risen with increasing nutrient levels (http://www.luontoportti.com/suomi/en/kalat/roach). Therefore,
roach may play an important role in the energy flow and nutrient cycling of a
eutrophic lake (Riemann et al., 1986). For example, the biomass of fish
composed mainly of roach and smelt in the Lake Vesijärvi, located in southern
Finland, is one of the main factors in maintaining high algal productivity and
biomass in the lake; and preferable for predicting food chain dynamics and
ecosystem responses in the littoral areas where densities of roach are greatest
(Horppila and Kairesalo, 1990). Roach are eurythermic fishes, tolerate wide
range of temperatures, with the ultimate upper lethal temperature of 33.5°C 
(Cocking, 1959). There is only little commercial fishing for roach fish,
but valued for recreational fishing.

 

 

1.2    Fish heart and Cardiovascular system

                                                               

As vertebrates, heart is an essential
organ in the cardivascular system and responsible for the delivery of oxygen
and nutrients to different tissues of the body depending on their energetics
needs and the removal of cellular metabolites and CO2 (Olson and
Farrell, 2006) . Fish heart traditionally descriped to be composed from four
chambers arranged in series: sinus venosus, atrium, ventricle and conus
arteriosus (or bulbus arteriosus). In teleost fish, the sinus venosus and bulbus
arteriosus mainly consist of connective tissue and are non-contractile, while
atrium and ventricle form the muscular pump (Santer, 1985; Satchell, 2008). Recently
studies revealed that the fish heart composed from six portions arranged in
series within the pericardium: sinus venosus, atrium, atrioventricular (AV) segment,
ventricle, conus arteriosus, and bulbus arteriosus. AV segment supports the AV
valves (ICardo and Colvee, 2011). Both of conus arteriosus and bulbus
arteriosus are gathered under the term of outflow tract (OFT) (Icardo, 2006). Conus
arteriosus constitutes a major part of the heart’s OFT in basal species, while
bulbus arteriosus constitutes the predominant portion of the OFT in most
teleosts (Icardo, 2017). As an exception, OFT components could not be distinguish
in the heart of Cyclostomata as lampreys (Kardong, 2006; Farrell, 2007; Icardo
et al., 2016). In addition, the AV region in lungfishes lacks any distinct
segmental characteristics (Icardo, 2017).

Teleost fishes
have closed cardiovasucular system. Paired ducts of Cuvier and hepatic veins transfer
the deoxygentaed venous blood to the thin-walled sinus venosus, acts as a
drainage pool for the venous system before the blood moves into the atrium, the
thin-walled muscular chamber (Santer, 1985; Satchell, 2008). Between the atrium
and the ventricle is the AV region, a ring of cardiac tissue plays an essential
role in regulating the conduction of AP (ICardo and Colvee, 2011). Atrium
contraction pushes the blood to fill the ventricle, the thicker-walled and
force-generating muscular chamber. Ventricle contracts and propel the blood to
the OFT leading into the beginning of the ventral aorta which transfers blood
to the gills for oxygentaion and then passes to all tissues of the body (Santer,
1985; Olson and Farrell, 2006; Genge et al., 2012). Circulation of blood
provides rapid convective transfer of oxygen, nutrients, hormones and
temperature all over the body. Sino-atrial, atrio-ventricular and
bulbo-ventricular valves prevent the reversed direction of blood during the
blood circulation.

                                               

1.3    electrical excitability of fish Heart

 

The heart acts as a muscular pump in the
circulatory system, which powers the movement of blood. Pump function of the
heart is based on regular and ordered sequence of contractions of the atrium
and the ventricle, which propel blood through the vasculature. Heart
contraction is initiated by electrical excitation of the sarcolemmal (SL) of
cardiac myocytes as a cardiac action potential (AP), which spreads all over
cardiomyocytes of the heart and contracts via increases in intracellular free
Ca2+ concentration (Cai) (Coraboeuf, 1978; Bers, 2000).
The sequence of events starting from the depolarization of SL to force production
by myofilaments via changes in Cai is called
excitation-contraction (EC) coupling and includes a number of ion channels, ion
pumps and ion transporters and their regulation by second messenger systems
(Fabiato, 1983; Tibbits et al., 1992; Vornanen et al., 2002; Bers, 2002).  

Electrical
excitability of the heart determines the rate and rhythm of cardiac contraction
under varying physiological states of the animal. Electrical excitability of
cardiac myocytes is produced by electrochemical gradients of Na+, K+
and Ca2+ ions across the SL. These cations and voltage difference unequally
distributed across the SL and provide the driving force for entry or exit of ions
through ion channels, which are gated to open by changes in membrane potential
or by binding of external ligands to the channel proteins (Bezanilla, 2005).
Carefully orchestrated opening and closing of several ion channels result in
generation of chamber-specific AP that travels from the site of origin over the
heart and causes sequential contraction of atrial and ventricular chambers by
triggering the release of Ca2+ ions from the intracellular stores of
the sarcoplasmic reticulum (Coraboeuf, 1978; Noble, 1984; Maltsev et al.,
2006). Electrical excitability of the cardiac myocyte is dependent on the
function of Na+-, K+- and Ca2+-specific ion
channels, integral membrane proteins, or protein assemblies of the cardiac SL.
Each of nodal, atrial, and ventricular tissues has special electrophysiological
characteristics, and therefore, different ion channel compositions.

                                                      

 

1.4    Action potential

 

Cardiac action potential (AP) initiates
and coordinates the contraction of the heart through the co-operation in
opening and closing of voltage- and ligand-gated ion channels of myocyte SL
(Bers, 2001). Ion current densities and channel compositions reveal regional
specialization in the heart, i.e. each cardiac chamber has distinct and
functionally AP. Pacemaker APs,  initiate
the spontaneous beating of the heart, are characterized by a slow
depolarization of membrane potential during diastole. Contrary, atrial and
ventricular APs have a clearly stable resting membrane potential (RMP) about
-80 mV with a fast depolarization. There is a striking difference between
atrial and ventricular AP duration, where atrial AP duration is shorter than
the ventricular AP (Saito and Tenma, 1976; Haverinen and Vornanen, 2009; Lin et
al., 2014; Vornanen, 2016). Changes in membrane voltage assosiated with the AP require
electrical charges to be transferred through the specific ion channels of
myosyte SL which mainly controled by Na+, Ca2+ and K+
ions.

As other
verteprates, fish cardiac AP can be divided into 5 phases (0-4) (Fozzard, 1977;
Roden et al., 2002; Vornanen, 2016). A resting cardiomyocyte has a stable
resting membrane potential (RMP) of -70 to -90 mV in atrial and ventricular
myocytes relative to the extracellular fluid (phase 4) which maintained by a efflux
of K+ ions through specific SL potassium channels. The resting case
broken by a voltage spread from neighboring myocytes (depolarization) and excite
the resting myocyte for contraction. After depolarization, RMP moves toward
positive voltages (threshold potential; TP), where the density of the inward Na+
current exceeds the total density of the outward K+ currents, a fast
upstroke of the AP is generated and within a few ms the membrane is shifted
from the threshold potential to a voltage above the zero level (phase 0). Phase
0 is produced by Na+ influx into the myocyte through SL Na channels.
the density of Na+ current is the major factor in determining the
rate of AP propagation over atrial and ventricular myocytes, i.e., the larger
Na+ influx causes the fatser AP upstroke. AP upstroke is followed by
rapid reploarization (phase 1) which generated by the transient outward K+
current. In contrast with mammalian cardiac APs, phase 1 is often inconspecious
or absent in fish cardiac myocytes. The
longest stage of AP (phase 2) is called “plateau phase” where the balance
between inward Ca2+ current and outward K+ current
occurred. Phase 2 is essential for Ca2+ influx and cardiac
contraction, and prevents
the heart from beating prematurely by delaying the recovery of Na+
channels from inactivated state. The fast restoration of the negative
RMP starts at the end of the plateau phase (phase 3), which produced by various
outward K+ currents (delayed rectifier K+ current, inward
rectifier K+ current and slow component of delayed rectifier K+
current). Restoration of RMP is crucial for the heart in diastole phase and
allows blood to fill the atrium and ventricle. Finally the
complete restoration of the negative RMP (phase 4).

Atrial
and ventricular APs have the same 5 phases of cardiac AP in the fish heart.
However, there is a significant difference between atrial and ventricular
myocytes in AP duration, where atrial AP is much shorter than ventricular AP
(Saito and Tenma, 1976; Haverinen and Vornanen, 2009; Lin et al., 2014). In
contrast to the stable RMP of atrial and ventricular myocytes, pacemaker APs
are characterized by gradual and slow diastolic depolarization towards the
threshold voltage of the AP upstroke (Saito, 1969; Saito, 1973; Harper et al.,
1995; Haverinen and Vornanen, 2007; Tessadori et al., 2012). Amplitude and
duration of the pacemaker AP is smaller and the rate of upstroke and
repolarization slower than those of atrial and ventricular APs. Transmission of
pacemaker APs to the neighboring atrial myocytes depolarizes them and starts
the spread of electrical excitation throughout the heart.

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