A trout swimming within a heart made of water, symbolizing unique cardiac research.

Trout Hearts: Unlocking Secrets for Better Heart Health?

Samir D’Costa in Science & Nature September 2025 • 4 min read.

"Could studying the unique heart rhythms of trout lead to new insights in human cardiology? Explore how a recent study challenges assumptions about heart function and pacemaker cells."

For decades, scientists have worked to unravel the complexities of the heart, that vital organ responsible for keeping us alive. Central to this quest is understanding the mechanisms that regulate heart rate, especially within pacemaker cells, which dictate the rhythm of our heartbeats. Conventional wisdom suggests a key role for what's known as the "funny current" (If), a unique electrical current within heart cells activated by hyperpolarization. This current is formed by hyperpolarization-activated HCN channels, and thought to be crucial in membrane clock regulation.

However, the role of If isn't quite so clear-cut in all creatures, particularly in the aquatic realm. Fish, for example, depend heavily on heart rate (fH) to adapt to changing temperatures, yet how their hearts regulate this rate remains somewhat of a mystery. A recent study published in the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology (August 30, 2017) throws a wrench into the gears of our understanding by examining the hearts of brown trout (Salmo trutta fario).

These scientists investigated the presence and function of HCN channels – the drivers of If – in the sinoatrial (SA) pacemaker cells of trout. Their findings challenge the long-held assumption that If is indispensable for regulating heart rate, opening up intriguing new avenues for research and a better comparative and comprehensive understanding of cardiac function.

Do Trout Hearts Beat to a Different Drum?

A trout swimming within a heart made of water, symbolizing unique cardiac research.

The research team, led by Minna Hassinen, Jaakko Haverinen, and Matti Vornanen at the University of Eastern Finland, delved deep into the inner workings of trout hearts to uncover the expression and function of HCN channels. Here's a breakdown of their approach:

The research methodology included:

Detailed Analysis of HCN Channel Transcripts: Researchers identified six different HCN channel transcripts (HCN1, HCN2a, HCN2ba, HCN2bb, HCN3, and HCN4) present in the trout heart tissue. They found that HCN3 and HCN4 were most abundant in the sinoatrial (SA) pacemaker cells which dictates the rate of beating.
Electrophysiological Experiments: Using patch-clamp techniques, the team measured the “funny current” (If) in isolated pacemaker cells, atrial myocytes, and ventricular myocytes.
Heterologous Expression: To further understand the function of trout HCN channels, specifically HCN3 and HCN4, these were expressed in Chinese hamster ovary (CHO) cells to observe their activity.
Spontaneous Beating Rate Measurement: The team recorded the beating rate of excised SA pacemaker tissue in the presence and absence of cesium (Cs+), a known If inhibitor, to assess If's influence on heart rate.

Now, for the juicy part, what did they actually discover?

A New Perspective on Heart Rhythms

The findings from this study encourage scientists to continue exploring the nuanced mechanisms that control heart function across different species. By challenging established dogmas, this research paves the way for new approaches to understanding and treating heart conditions. While trout hearts may seem far removed from human cardiology, they offer valuable clues that could one day lead to innovative therapies.

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Everything You Need To Know

What is the main focus of the study on trout hearts and its potential implications?

The primary focus of the study is to understand how the hearts of brown trout (*Salmo trutta fario*) regulate their heart rate (fH), especially within the sinoatrial (SA) pacemaker cells. The research challenges the conventional understanding of the role of the "funny current" (If) and HCN channels in heart rhythm regulation. The implications of this research could lead to innovative therapies for heart conditions in humans by offering new perspectives on cardiac function, which is a critical step in unraveling the complexities of the human heart and its potential treatment strategies. These findings suggest that mechanisms controlling heart function may vary across species, prompting further investigation into these nuanced differences and similarities.

What are the specific methods used to analyze the heart function of trout, and what did they reveal about the "funny current"?

The study employed several methods to investigate the trout heart. Researchers analyzed the presence and function of HCN channels, which are the drivers of the "funny current" (If). The methodology included identifying six different HCN channel transcripts (HCN1, HCN2a, HCN2ba, HCN2bb, HCN3, and HCN4) in trout heart tissue. The team measured the If in isolated pacemaker cells, atrial myocytes, and ventricular myocytes using patch-clamp techniques. They also used heterologous expression, specifically in Chinese hamster ovary (CHO) cells, to understand the function of trout HCN channels like HCN3 and HCN4. Furthermore, the team recorded the spontaneous beating rate of excised SA pacemaker tissue with and without cesium (Cs+), an If inhibitor, to assess If's influence on heart rate. The research team discovered that the role of If might not be as indispensable in heart rate regulation in trout as previously believed, challenging existing assumptions regarding the importance of the "funny current".

How does the study on trout hearts challenge the existing understanding of heart function regulation?

The study challenges the long-held assumption that the "funny current" (If), generated by hyperpolarization-activated HCN channels, is indispensable for regulating heart rate. Traditionally, If has been considered crucial for the membrane clock regulation within pacemaker cells. However, the findings indicate that this may not be the sole or primary mechanism in brown trout. The research suggests that the mechanisms controlling heart function can vary across species, indicating a more complex and nuanced understanding of heart rhythm regulation is required. This challenges the established view and encourages scientists to explore alternative or additional mechanisms involved in cardiac function.

What role do HCN channels play in the context of trout heart research, and why are they significant?

HCN channels, specifically the different transcripts identified in the trout heart (HCN1, HCN2a, HCN2ba, HCN2bb, HCN3, and HCN4), are significant because they are the drivers of the "funny current" (If). The study focused on these channels to understand their function in regulating the heart rate (fH) of brown trout. The abundance of HCN3 and HCN4 in the sinoatrial (SA) pacemaker cells suggests a key role in the rhythm of the trout heart. By examining the function of these channels, the researchers aimed to understand the role of If, and how it contributes to the heart's rhythm, challenging the current understanding of If's importance and revealing the potential for different regulatory mechanisms in aquatic species.

In what ways could the findings from the trout heart study potentially impact human cardiology and future research?

The study on trout hearts could significantly impact human cardiology by providing new insights into the mechanisms of heart rhythm regulation. By challenging established dogmas about the role of the "funny current" (If) and HCN channels, the research paves the way for new approaches to understanding and treating heart conditions. The findings suggest that mechanisms controlling heart function can vary across species, prompting further investigation into these differences. This comparative approach could reveal novel therapeutic targets or strategies for human heart health. For example, understanding the unique adaptations in trout hearts, such as alternative mechanisms to regulate heart rate, might inspire new therapies for human conditions where the "funny current" is not functioning optimally or when different approaches are needed to support heart health.